core: link user keyring to session keyring (#6275)

Commit  74dd6b515f (core: run each system
service with a fresh session keyring) broke adding keys to user keyring.
Added keys could not be accessed with error message:

keyctl_read_alloc: Permission denied

So link the user keyring to our session keyring.

src/basic/missing.h

@@ -1093,6 +1093,10 @@ typedef int32_t key_serial_t;
 #define KEYCTL_DESCRIBE 6
 #endif
 
+#ifndef KEYCTL_LINK
+#define KEYCTL_LINK 8
+#endif
+
 #ifndef KEYCTL_READ
 #define KEYCTL_READ 11
 #endif