* add support for reading the UID via getsockopt from unix domain socket conneciotns...

* add support for reading the UID via getsockopt from unix domain socket conneciotns in begin_session()
* use the UID like bzed suggested in debian Bug#528760 when sending mails via LMTP
* add configure options to detect ucread; borowed from http://git.musicpd.org/cgit/master/mpd.git/plain/m4/ucred.m4; thanks to Max Kellermann to point me there.
a further good read is http://www.wsinnovations.com/softeng/articles/uds.html ; it indicates that this might not be portable to *bsd / Solaris; configure should protect us from problems here though.