Provide more macros for handling of interned strings

 * str_erealloc behaves like erealloc for normal strings, but will
   use emalloc+memcpy for interned strings.
 * str_estrndup behaves like estrndup for normal strings, but will
   not copy interned strings.
 * str_strndup behaves like zend_strndup for normal strings, but
   will not copy interned strings.
 * str_efree_rel behaves like efree_rel for normal strings, but
   will not free interned strings.
 * str_hash will return INTERNED_HASH for interned strings and
   compute it using zend_hash_func for normal strings.
Fix two warnings