bugfix: postpone the start of the imgssapi listener

If the listen socket is opened earlier (during configuration parsing),
it is closed again during early init.

The start has been postponed to the 'runInput' stage, which has an
unpleasant side effect that it will fail if the priviledges are
dropped.

The code should be eventually fixed to support the new config system.
doc: clarify RSYSLOG_DEBUG format
doc: correct omfile documentation

Some of the options affect all files, not just dynafiles.
build: don't check for libs that are not needed