userdbd: simplify logic for generating NSS listings

So far we basically had two ways to iterate through NSS records: one via
the varlink IPC and one via the userdb.[ch] infra, with slightly
different implementations.

Let's clean this up, and always use userdb.[ch] also when resolving via
userdbd. The different codepaths for the NameServiceSwitch and the
Multiplexer varlink service now differ only in the different flags
passed to the userdb lookup.

Behaviour shouldn't change by this. This is mostly refactoring, reducing
redundant codepaths.

src/shared/userdb.h

@@ -24,6 +24,9 @@ typedef enum UserDBFlags {
         USERDB_EXCLUDE_DYNAMIC_USER = 1 << 4,  /* exclude looking up in io.systemd.DynamicUser */
         USERDB_AVOID_MULTIPLEXER    = 1 << 5,  /* exclude looking up via io.systemd.Multiplexer */
         USERDB_DONT_SYNTHESIZE      = 1 << 6,  /* don't synthesize root/nobody */
+
+        /* Combinations */
+        USERDB_NSS_ONLY = USERDB_EXCLUDE_VARLINK|USERDB_DONT_SYNTHESIZE,
 } UserDBFlags;
 
 int userdb_by_name(const char *name, UserDBFlags flags, UserRecord **ret);