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/bltins/test.c

@@ -666,7 +666,7 @@ skip:
 				if((maxgroups=getgroups(0,(gid_t*)0)) <= 0)
 				{
 					/* pre-POSIX system */
-					maxgroups=NGROUPS_MAX;
+					maxgroups = (int)astconf_long(CONF_NGROUPS_MAX);
 				}
 			}
 			groups = (gid_t*)stakalloc((maxgroups+1)*sizeof(gid_t));
@@ -680,7 +680,7 @@ skip:
 				}
 			}
 		}
-#   endif /* _lib_getgroups */
+#endif /* _lib_getgroups */
 		if(statb.st_mode & mode)
 			return(0);
 	}