Don't be silent when going background and logger fails

Doing box.cfg{background=true, logger='/EACCESS.log'} resulted in a
silent failure because initialization routine closed stderr first before
initializing the logger.

Changing stuff to absolutize pathes (logger, pid_file) to make it easier
to troubleshoot errors.

Things in say.c (logger guts) were changed to call exit instead of _exit
on failure in order for the cleanup to be performed.

Also adding a library for managing pid files under
third_party/libutil_freebsd.