EbuildIpcDaemon: use non-blocking write

EbuildIpcDaemon: use non-blocking write

This prevents the Scheduler from hanging if the client is killed before
we can send the reply, as reported by David James:

  http://codereview.chromium.org/6713003

This commit depends on ebuild-ipc (the client) opening the other side
of this fifo before it sends its request, which has already been added
in commit eff879ff0ce7dcc1ce68d5f16de1ec73051f8c18.