wayland: Bump RLIMIT_NOFILE limit

As a wayland compositor kwin can hold many open file descriptors and
therefore it's easier for it to hit the soft file descriptor limit. This
change makes kwin_wayland bump the soft file descriptor limit to the
hard limit. With systemd, it's usually 500K.

In order to prevent causing mess in child processes, pthread_atfork() is
used to register a handler which will be invoked in the child process
after fork() in order to restore RLIMIT_NOFILE to its original value.