MEDIUM: startup: move PID handling in init()

MEDIUM: startup: move PID handling in init()

Let's move PID handling in init() from the main() code. It is more appropriate
to open and to write the PID of the process just after daemonization fork. In
case of daemon monoprocess mode, we will simply write a PID of the process,
which is already in the background. In case of 'master-worker' mode, we keep
the previous behaviour and we write only a PID of the master process.

This allows to remove redundant tests of the process execution mode, tests of
the pidfd value and consequent writes to this pidfd. This patch prepares the
refactoring of master-worker fork by moving it in init() function as well.