This allows to simplify net_real_read() and net_real_write() a bit.

Removed some superfluous #ifdef/ifndef MYSQL_SERVER from net_serv.cc
The code always runs in server, either normal or embedded.
Dead code for switching socket between blocking and non-blocking modes,
is also removed.

Removed pthread_kill() with alarm signal that woke up main thread on
server shutdown. Used shutdown(2) on polling sockets instead, to the same
effect.

Removed yet another superstitious pthread_kill(), that ran on non-Windows
in terminate_slave_thread().