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/cmd/ksh93/features/externs

@@ -46,7 +46,7 @@ tst	note{ determining extra bytes per argument for arguments list }end output{
 			for(i=0; environ[i]; i++)
 				envlen += strlen(environ[i]) + 1 + extra_bytes;
 			envlen += 1 + extra_bytes;	/* final null element */
-			argmax = strtoimax(astconf("ARG_MAX",NiL,NiL),NiL,0) - envlen;
+			argmax = (int)astconf_long(CONF_ARG_MAX) - envlen;
 			if (argmax < 2048)
 			{
 				error(ERROR_ERROR|2, "argmax too small");