command -x: fix efficiency; always run external cmd (re: acf84e96)

This commit fixes 'command -x' to adapt to OS limitations with
regards to data alignment in the arguments list. A feature test is
added that detects if the OS aligns the argument on 32-bit or
64-bit boundaries or not at all, allowing 'command -x' to avoid
E2BIG errors while maximising efficiency.

Also, as of now, 'command -x' is a way to bypass built-ins and
run/query an external command. Built-ins do not limit the length of
their argument list, so '-x' never made sense to use for them. And
because '-x' hangs on Linux and macOS on every ksh93 release
version to date (see acf84e96), few use it, so there is little
reason not to make this change.

Finally, this fixes a longstanding bug that caused the minimum exit
status of 'command -x' to be 1 if a command with many arguments was
divided into several command invocations. This is done by replacing
broken flaggery with a new SH_XARG state flag bit.

src/cmd/ksh93/features/externs:
- Add new C feature test detecting byte alignment in args list.
  The test writes a #define ARG_ALIGN_BYTES with the amount of
  bytes the OS aligns arguments to, or zero for no alignment.

src/cmd/ksh93/include/defs.h:
- Add new SH_XARG state bit indicating 'command -x' is active.

src/cmd/ksh93/sh/path.c: path_xargs():
- Leave extra 2k in the args buffer instead of 1k, just to be sure;
  some commands add large environment variables these days.
- Fix a bug in subtracting the length of existing arguments and
  environment variables. 'size -= strlen(cp)-1;' subtracts one less
  than the size of cp, which makes no sense; what is necessary is
  to substract the length plus one to account for the terminating
  zero byte, i.e.: 'size -= strlen(cp)+1'.
- Use the ARG_ALIGN_BYTES feature test result to match the OS's
  data alignment requirements.
- path_spawn(): E2BIG: Change to checking SH_XARG state bit.

src/cmd/ksh93/bltins/whence.c: b_command():
- Allow combining -x with -p, -v and -V with the expected results
  by setting P_FLAG to act like 'whence -p'. E.g., as of now,
	command -xv printf
  is equivalent to
	whence -p printf
  but note that 'whence' has no equivalent of 'command -pvx printf'
  which searches $(getconf PATH) for a command.
- When -x will run a command, now set the new SH_XARG state flag.

src/cmd/ksh93/sh/xec.c: sh_exec():
- Change to using the new SH_XARG state bit.
- Skip the check for built-ins if SH_XARG is active, so that
  'command -x' now always runs an external command.

src/lib/libcmd/date.c, src/lib/libcmd/uname.c:
- These path-bound builtins sometimes need to run the external
  system command by the same name, but they did that by hardcoding
  an unportable direct path. Now that 'command -x' runs an external
  command, change this to using 'command -px' to guarantee using
  the known-good external system utility in the default PATH.
- In date.c, fix the format string passed to 'command -px date'
  when setting the date; it was only compatible with BSD systems.
  Use the POSIX variant on non-BSD systems.

src/cmd/ksh93/sh/path.c

@@ -162,24 +162,24 @@ static pid_t path_xargs(Shell_t *shp,const char *path, char *argv[],char *const
 	pid_t pid;
 	if(shp->xargmin < 0)
 		return((pid_t)-1);
-	size = shp->gd->lim.arg_max-1024;
+	size = shp->gd->lim.arg_max-2048;
 	for(ev=envp; cp= *ev; ev++)
-		size -= strlen(cp)-1;
+		size -= strlen(cp)+1;
 	for(av=argv; (cp= *av) && av< &argv[shp->xargmin]; av++)  
-		size -= strlen(cp)-1;
+		size -= strlen(cp)+1;
 	for(av=avlast; cp= *av; av++,nlast++)  
-		size -= strlen(cp)-1;
+		size -= strlen(cp)+1;
 	av =  &argv[shp->xargmin];
 	if(!spawn)
 		job_clear();
 	shp->exitval = 0;
 	while(av<avlast)
 	{
-		/* for each argument, account for terminating zero and 16-byte alignment */
+		/* for each argument, account for terminating zero and possible alignment */
 		for(xv=av,left=size; left>0 && av<avlast;)
 		{
-			n = strlen(*av++) + 1 + 16;
-			left -= n + n % 16;
+			n = strlen(*av++) + 1 + ARG_ALIGN_BYTES;
+			left -= n + (ARG_ALIGN_BYTES ? n % ARG_ALIGN_BYTES : 0);
 		}
 		/* leave at least two for last */
 		if(left<0 && (avlast-av)<2)
@@ -1244,11 +1244,11 @@ pid_t path_spawn(Shell_t *shp,const char *opath,register char **argv, char **env
 #endif /* EMLINK */
 		return(-1);
 	    case E2BIG:
-		if(shp->xargmin)
+		if(sh_isstate(SH_XARG))
 		{
 			pid = path_xargs(shp,opath, &argv[0] ,envp,spawn);
 			if(pid<0)
-				errormsg(SH_DICT,ERROR_system(ERROR_NOEXEC),"%s: 'command -x' failed",path);
+				errormsg(SH_DICT,ERROR_system(ERROR_NOEXEC),"command -x: could not execute %s",path);
 			return(pid);
 		}
 	    default: