Compile english error messages into mysqld executable.

Currently, server startup requires message directory to be set and file to be found. It would greatly increase usability, if server was independent on the message file. We could compile English messages into the executable itself instead. Apart from usability improvement, and slightly reduced startup time, this would help with other tasks. xtrabackup for example, should do plugin loading like the server does, and would reuse server functionality. However any attempt to write error message from plugin_init() when it is called from xtrabackup, would crash if message file is not loaded. I'd like to avoid introducing additional -lc_messages_dir or similar parameters for xtrabackup,(not its core functionality)