check ulimit and warn if low

[kapouer]

When running an application using webkitgtk, it may happen that ulimit is set to a too low value.
This is bad because webkitgtk opens many (many) fds, so it will fail after some time.

Upon first require, webkitgtk should warn about that situation because it's not always obvious to know the real ulimit value of the process once it is running under systemd supervision.