libs/libc/pwd and libs/libc/grp: Modify to giet user/group data from /etc/passwd and /etc/group if so configurated.

Squashed commit of the following:

    Last minute clean-up
    libs/libc/grp/lib_find_grpfile.c:  Fix some problems found in testing.
    libs/libc/pwd:  Finishes off implementation using /etc/passwd.
    libs/libc/grp:  Finishes off implementation using /etc/group.
    libs/libc/pwd/lib_find_pwdfile.c:  Add logic to look up user information in /etc/passwd file, if available.
    libs/libc/grp/lib_find_grpfile.c:  Add logic to look up group information in /etc/group file, if available.

libs/libc/grp/lib_getgrgidr.c

@@ -72,6 +72,19 @@
 int getgrgid_r(gid_t gid, FAR struct group *grp, FAR char *buf, size_t buflen,
                FAR struct group **result)
 {
+#ifdef CONFIG_LIBC_GROUP_FILE
+  int ret;
+
+  ret = grp_findby_gid(gid, grp, buf, buflen);
+  if (ret != 1)
+    {
+      *result = NULL;
+      return ret < 0 ? -ret : 0;
+    }
+
+  *result = grp;
+  return 0;
+#else
   if (gid != ROOT_GID)
     {
       /* The only known group is 'root', which has a gid of 0.  Thus, report
@@ -84,4 +97,5 @@ int getgrgid_r(gid_t gid, FAR struct group *grp, FAR char *buf, size_t buflen,
 
   return getgrbuf_r(ROOT_GID, ROOT_NAME, ROOT_PASSWD, grp, buf, buflen,
                     result);
+#endif
 }