nss-mymachines: map userns users of containers to real user names

Given a container "foo", that maps user id $UID to container user, using
user namespaces, this NSS module extenstion will now map the $UID to a
name "vu-foo-$TUID" for the translated UID $UID.

Similar, userns groups are mapped to "vg-foo-$TGID" for translated GIDs
of $GID.

This simple change should make userns users more discoverable. Also,
given that many tools like "adduser" check NSS before allocating a UID,
should lower the chance of UID range conflicts between tools.

src/basic/macro.h

@@ -26,6 +26,7 @@
 #include <sys/types.h>
 #include <sys/uio.h>
 #include <inttypes.h>
+#include <stdbool.h>
 
 #define _printf_(a,b) __attribute__ ((format (printf, a, b)))
 #define _alloc_(...) __attribute__ ((alloc_size(__VA_ARGS__)))
@@ -461,6 +462,18 @@ do {                                                                    \
 #define GID_INVALID ((gid_t) -1)
 #define MODE_INVALID ((mode_t) -1)
 
+static inline bool UID_IS_INVALID(uid_t uid) {
+        /* We consider both the old 16bit -1 user and the newer 32bit
+         * -1 user invalid, since they are or used to be incompatible
+         * with syscalls such as setresuid() or chown(). */
+
+        return uid == (uid_t) ((uint32_t) -1) || uid == (uid_t) ((uint16_t) -1);
+}
+
+static inline bool GID_IS_INVALID(gid_t gid) {
+        return gid == (gid_t) ((uint32_t) -1) || gid == (gid_t) ((uint16_t) -1);
+}
+
 #define DEFINE_TRIVIAL_CLEANUP_FUNC(type, func)                 \
         static inline void func##p(type *p) {                   \
                 if (*p)                                         \