lib: Don't crash if getenv() is called after process_title_deinit() with Linux/OSX...

lib: Don't crash if getenv() is called after process_title_deinit() with Linux/OSX PROCTITLE_HACK

Right now the only known issue is GNUTLS's lib_deinit():

e = getenv("GNUTLS_NO_EXPLICIT_INIT");

But we likely never want that to be set. Alternative ways to solve this are
more annoying than our clearing the environment. Libraries probably shouldn't
rely on it much anyway, because some parts of Dovecot clear the whole
environment while running.