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/libast/include/ast.h

@@ -283,6 +283,27 @@ extern off_t		astcopy(int, int, off_t);
 extern int		astlicense(char*, int, char*, char*, int, int, int);
 extern int		astquery(int, const char*, ...);
 extern void		astwinsize(int, int*, int*);
+#if _lib_sysconf
+/* prefer sysconf for astconf_long and astconf_ulong to improve performance */
+#define CONF_ARG_MAX		_SC_ARG_MAX
+#define CONF_CHILD_MAX		_SC_CHILD_MAX
+#define CONF_CLK_TCK		_SC_CLK_TCK
+#define CONF_NGROUPS_MAX	_SC_NGROUPS_MAX
+#define CONF_OPEN_MAX		_SC_OPEN_MAX
+#define CONF_PAGESIZE		_SC_PAGESIZE
+#define astconf_long(x)		sysconf(x)
+#define astconf_ulong(x)	(unsigned long)sysconf(x)
+#else
+/* fallback in case sysconf isn't available */
+#define CONF_ARG_MAX		"ARG_MAX"
+#define CONF_CHILD_MAX		"CHILD_MAX"
+#define CONF_CLK_TCK		"CLK_TCK"
+#define CONF_NGROUPS_MAX	"NGROUPS_MAX"
+#define CONF_OPEN_MAX		"OPEN_MAX"
+#define CONF_PAGESIZE		"PAGESIZE"
+#define astconf_long(x)		strtol(astconf(x,NiL,NiL),NiL,0)
+#define astconf_ulong(x)	strtoul(astconf(x,NiL,NiL),NiL,0)
+#endif
 
 extern ssize_t		base64encode(const void*, size_t, void**, void*, size_t, void**);
 extern ssize_t		base64decode(const void*, size_t, void**, void*, size_t, void**);