Moved credential-related code to nxt_credential.c.

This is required to avoid include cycles, as some nxt_clone_* functions
depend on the credential structures, but nxt_process depends on clone
structures.

src/nxt_runtime.h

@@ -58,7 +58,7 @@ struct nxt_runtime_s {
     const char             *engine;
     uint32_t               engine_connections;
     uint32_t               auxiliary_threads;
-    nxt_user_cred_t        user_cred;
+    nxt_credential_t       user_cred;
     nxt_capabilities_t     capabilities;
     const char             *group;
     const char             *pid;