Centralise handling & ignoring of SIGPIPE in qcore_unix_p.h

Centralise handling & ignoring of SIGPIPE in qcore_unix_p.h

We had two instances of this function in the Qt source code, one clearly
a copy of the other, so both had the same thread-safety issue. Instead,
let's have one copy and have both write_nosignal() and sendto() call
them.

Q_NO_POSIX_SIGNALS is also gone. It was only used with Symbian.

Change-Id: I0f1354a8e9df8e6b10a02f86a940e3c6d1222087
Reviewed-by: Peter Hartmann <phartmann@rim.com>
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>