udp_send: Silence gcc warning.

udp_send: Silence gcc warning.

Ubuntu Lucid's gcc-4.3.3 emits the following warning about not checking
the return value of write() even if the result is casted to void.

	udp_send.c: In function ‘udp_close_target’:
	udp_send.c:65: warning: ignoring return value of ‘write’, declared with attribute warn_unused_result

The usual approach to store the return value in a dummy variable is
not optimal because the upcoming gcc-4.6 will warn that the variable
is set but otherwise unused.

Likely there are more places in the paraslash code which have the same
problem, so this patch introduce the do_nothing macro in para.h which,
creatively enough, does nothing. The new macro is employed like

	if (write(...))
		do_nothing;

This gets rid of the warning on Lucid and does not give new warning
on gcc-4.6.