vgcreate: improve checks for existing global lock

This tries harder to avoid creating duplicate global locks in
sanlock VGs by refusing to create a new sanlock VG with a
global lock if other sanlock VGs exist that may have a gl.

lib/cache/lvmcache.h

@@ -188,4 +188,6 @@ int lvmcache_found_duplicate_pvs(void);
 
 void lvmcache_set_preferred_duplicates(const char *vgid);
 
+int lvmcache_contains_lock_type_sanlock(struct cmd_context *cmd);
+
 #endif