MEDIUM: startup: do set_identity() if needed in one place

MEDIUM: startup: do set_identity() if needed in one place

There are two set_identity() calls, both under quite same:

    'if ((global.mode & (MODE_MWORKER|MODE_DAEMON...)...'

The first call serves to change uid/gid and set some needed Linux capabilities
only for process in the foreground mode. The second comes after master-worker
fork and allows to do the same in daemon and in worker modes.

Due to moving the master-worker fork in init() in some previous commit, the
second set_identity() now is no longer under the 'if'. So, it is executed
for all modes, except MODE_MWORKER. Now in MODE_MWORKER process enters in its
wait polling loop just after forking a worker and it terminates almost
immediately, if it exits this loop.

Worker, daemon and process in a foreground mode will perform set_identity() as
before, but now it will be called in a one place at main().

global.last_checks should be verified just after set_identity() call. As it's
stated in comments some configuration options may require full privileges or
some Linux capabilities need to be granted to process. set_identity() via
prepare_caps_for_setuid() may put configured capabilities in process Effective
set and, hence, remove respective flag from global.last_checks.