merge: convert Buffers to use the pool

merge: convert Buffers to use the pool

 * remove unused Buffer in cs_inherit_variable()
 * allocate Buffer in ac_free()
 * allocate Buffer in cs_register_variables()
 * allocate Buffer in mutt_folder_hook()
 * allocate Buffer in lua_mutt_call()
 * allocate Buffer in lua_mutt_enter()
 * allocate Buffer in mutt_find_hook()
 * allocate Buffer in mutt_account_hook()
 * allocate Buffer in execute_commands()
 * allocate Buffer in mutt_dump_variables()