Add simple reference-counted string allocator and use it for passing

around references to the sudoers path.  This lets us avoid making
copies of the sudoers path for the errorfile as well as each Defaults
entry.

plugins/sudoers/sudoers.h

@@ -403,4 +403,10 @@ bool sudoers_gc_add(enum sudoers_gc_types type, void *ptr);
 bool sudoers_gc_remove(enum sudoers_gc_types type, void *ptr);
 void sudoers_gc_init(void);
 
+/* rcstr.c */
+char *rcstr_dup(const char *src);
+char *rcstr_alloc(size_t len);
+char *rcstr_addref(const char *s);
+void rcstr_delref(const char *s);
+
 #endif /* SUDOERS_SUDOERS_H */