Avoid compiler warnings due to ignored write return values

Glibc (at least) provides the warn_unused_result attribute on write,
(if optimizing and _FORTIFY_SOURCE is defined). So we explicitly
ignore the return value in our signal handler, where we couldn't do
anything anyway.

Compile with:

	make CFLAGS="-O -D_FORTIFY_SOURCE"

before this commit to see the warning.