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

... as well as some other minor issues.

The imdiag module now can very effectively inject messages, which also
frees us from uncertainties of tcp reception and processing. All shell
script based tests have been modularized, what makes it far easier to
create new tests. Also, the test bench now executes more reliable and
much faster, because we can now rely on actual engine information where
we previously did just a dumb sleep.

which enables to talk to the rsyslog core at runtime. The current
implementation is only a beginning, but can be expanded over time

imdiag was never finished (not even really begun), but now I need it.
I made the few things that are available compile, but more serious
work is required.

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.