rm redundant hardcoded default paths (re: aa4669ad)

As of aa4669ad, astconf("PATH") is implemented as a hardcoded AST
configuration variable that always has a value, instead of one that
falls back on the OS. Its value is now obtained from the OS (with a
fallback) at configure time and not at runtime. This means that any
fallback for astconf("PATH") is now never used.

src/cmd/ksh93/data/msg.c,
src/cmd/ksh93/include/shell.h:
- Remove e_defpath[]. (The path "/bin:/usr/bin:" made no sense as a
  default path anyway, as the final empty element is wrong: default
  utilities should never be sought in the current working dir.)

src/cmd/ksh93/sh/path.c,
src/lib/libast/path/pathbin.c:
- abort() if astconf("PATH") returns null.

src/lib/libast/comp/conf.tab: PATH:
- If no 'getconf' utility can be found, use a fallback path that
  finds more utilities by also searching in 'sbin' directories.
  On some systems, this is needed to find chown(1).

src/cmd/ksh93/sh.1:
- Update doc re default path.

src/lib/libast/comp/conf.tab

@@ -375,7 +375,7 @@ PATH				AST	CS 1 MU		sh{
 	*)	CONF_path=`"$CONF_getconf" PATH` ;;
 	esac
 	case $CONF_path in
-	'')	echo '"/bin:/usr/bin"' ;;
+	'')	echo '"/bin:/usr/bin:/sbin:/usr/sbin"' ;;
 	*)	echo "\\"$CONF_path\\"" ;;
 	esac
 }

src/lib/libast/path/pathbin.c

@@ -39,7 +39,7 @@ pathbin(void)
 	if ((!(bin = getenv("PATH")) || !*bin) && !(bin = val))
 	{
 		if (!*(bin = astconf("PATH", NiL, NiL)) || !(bin = strdup(bin)))
-			bin = "/bin:/usr/bin:/usr/local/bin";
+			abort();
 		val = bin;
 	}
 	return bin;