Use EINVAL instead of EDOOFUS. Add a missing synopsis for
pidfile_fileno() in the man page. Move the definition of struct pidfh
from libutil.h into pidfile.c following upstream change.
We have moved all man pages to section 3bsd, let's do the same for the
page title, and in all the references for uniformity and to avoid
confusions.
Fixes: https://bugs.freedesktop.org/101545