REORG: startup: move mworker_run_master and mworker_loop in mworker.c

REORG: startup: move mworker_run_master and mworker_loop in mworker.c

mworker_run_master() is called only in master mode. mworker_loop() is static
and called only in mworker_run_master(). So let's move these both functions in
mworker.c.

We also need here to make run_thread_poll_loop() accessible from other units,
as it's used in mworker_loop().