Feature: allow changing the identifier for syslog (+tests)

Original "qb_log_ctl" interface had to be extended for passing read-only
strings (new parameter), resulting in new "qb_log_ctl2" function, which
is what qb_log_ctl calls into with the new parameter set to NULL.
This ensures backward compatibility.

A new QB_LOG_CONF_IDENT configuration directive for the mentioned
interface is added with a goal to set new internal identifier
that is, notably, used for syslog sink. This allows for switching
the identification without a need to reinitialize logging subsystem,
akin to changing target logging facility.

Also a brand new concept of testing syslog sink in particular is
introduced (finally). During initial trial&error stage, it used
LD_PRELOAD hack but it seems that libtool is sophisticated enough
that no such extra intervention is needed and the desired symbol
resolution Just Works (tm). However, the technique is highly
non-portable (there is even a warning about that from libtool,
which is partially on purpose as the _syslog_override.so should
rather be explicit it is by no mean a regular library) and hence
the syslog tests have to be enabled with explicit

./configure --enable-syslog-tests

rather than possibly break on untested platforms (far too many).
The concept can be extended upon, but initially, just the new
feature is being tested.

Post-review: thanks Chrissie for a suggestion how to deal with
extract-arg-and-forget in a less intrusive way (no defines).