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/xec.c

@@ -1024,7 +1024,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 			}
 #endif /* SHOPT_NAMESPACE */
 			com0 = com[0];
-			shp->xargexit = 0;
+			sh_offstate(SH_XARG);
 			while(np==SYSCOMMAND || !np && com0 && nv_search(com0,shp->fun_tree,0)==SYSCOMMAND)
 			{
 				register int n = b_command(0,com,&shp->bltindata);
@@ -1036,13 +1036,12 @@ int sh_exec(register const Shnode_t *t, int flags)
 					break;
 				np = nv_bfsearch(com0, shp->bltin_tree, &nq, &cp); 
 			}
-			if(shp->xargexit)
+			if(sh_isstate(SH_XARG))
 			{
 				shp->xargmin -= command;
 				shp->xargmax -= command;
+				shp->xargexit = 0;
 			}
-			else
-				shp->xargmin = 0;
 			argn -= command;
 			if(np && is_abuiltin(np))
 			{
@@ -1233,7 +1232,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 					pipejob = 1;
 				}
 				/* check for builtins */
-				if(np && is_abuiltin(np))
+				if(np && is_abuiltin(np) && !sh_isstate(SH_XARG))
 				{
 					volatile int scope=0, share=0;
 					volatile void *save_ptr;