- saved gettimeofday() calls in imtcp (and increased reception buffer)
- somewhat optimized stringbuf.c
- some other optimizations

Conflicts:
	tests/Makefile.am

Conflicts:
	tests/Makefile.am

Well, actually this and a lot of related things. I improved the
testbench so that the new capabilities are automatically tested and
also did some general cleanup. The current multiple tcp listener
solution will probably receive some further cleanup, too, but looks
quite OK so far. I also reviewed the way tcpsrv et all work, in
preparation of using this code for imdiag. I need to document the
findings, especially as the code is rather complicated "thanks" to
the combination of plain tcp and gssapi transport modes.

Thanks to Michael Biebl for helping to debug this one.

Conflicts:
	ChangeLog
	Makefile.am
	configure.ac
	doc/manual.html

pkg-config uses uppercase for its _CFLAGS and _LIBS variable names.
So we follow suit and convert all _cflags and _libs variables to
uppercase for more consistency.

Conflicts:
	ChangeLog
	configure.ac
	doc/manual.html
	plugins/imudp/imudp.c

...tried to send a message to it. This behaviour is operator-configurable.
If enabled, a message was emitted each time. That way an attacker could
effectively fill the disk via this facility. The message is now
emitted only once in a minute (this currently is a hard-coded limit,
if someone comes up with a good reason to make it configurable, we
will probably do that).