Use dynamic maximum configuration values when necessary (#370)

This commit fixes an issue with how ksh was obtaining the value of
NGROUPS_MAX. On some systems this setting can be changed (e.g., on
illumos adding 'set ngroups_max=32' to /etc/system then rebooting
changes NGROUPS_MAX from 16 to 32). Ksh was using NGROUPS_MAX with
the assumption it's a static value, which could cause issues on
systems where it isn't static. This bugfix is inspired by the one
from <b1362c3a5>, although it
has been expanded a bit to account for OPEN_MAX as well.

src/cmd/ksh93/sh/init.c, src/lib/libcmd/fds.c:
- Rename the getconf() macro to astconf_long() and move it to ast.h
  to prevent redundancy. Other sections of the code have been
  modified to use this macro for astconf() to account for
  dynamic settings.
- An equivalent macro for unsigned long values (astconf_ulong) has
  been added.
- Prefer sysconf(3) where available. It has better performance as it
  returns a numeric value directly instead of via string
  conversion.
- The astconf_long and astconf_ulong macros have been documented in
  the ast(3) man page.

src/lib/libcmd/fds.c

@@ -54,9 +54,6 @@ static const char usage[] =
 #define major(x)	(int)(((unsigned int)(x)>>8)&0xff)
 #endif
 
-#undef	getconf
-#define getconf(x)	strtol(astconf(x,NiL,NiL),NiL,0)
-
 #ifdef S_IFSOCK
 
 typedef struct NV_s
@@ -214,8 +211,14 @@ b_fds(int argc, char** argv, Shbltin_t* context)
 		error(ERROR_usage(2), "%s", optusage(NiL));
 		UNREACHABLE();
 	}
-	if ((open_max = getconf("OPEN_MAX")) <= 0)
+	if ((open_max = (int)astconf_long(CONF_OPEN_MAX)) <= 0)
+	{
+#if defined(OPEN_MAX)
 		open_max = OPEN_MAX;
+#else
+		open_max = FOPEN_MAX;
+#endif
+	}
 	if (unit == 1)
 		sp = sfstdout;
 	else if (fstat(unit, &st) || !(sp = sfnew(NiL, NiL, SF_UNBOUND, unit, SF_WRITE)))

src/lib/libcmd/id.c

@@ -231,7 +231,7 @@ getids(Sfio_t* sp, const char* name, register int flags)
 			 */
 
 			if ((maxgroups = getgroups(0, groups)) <= 0)
-				maxgroups = NGROUPS_MAX;
+				maxgroups = (int)astconf_long(CONF_NGROUPS_MAX);
 			if (!(groups = newof(0, gid_t, maxgroups + 1, 0)))
 			{
 				error(ERROR_SYSTEM|ERROR_PANIC, "out of memory [group array]");