ebuild-ipc: use non-blocking read

ebuild-ipc: use non-blocking read

This makes it possible for the daemon to send a reply without blocking,
thereby improving performance and also making it possible for the
daemon to do a non-blocking write without a race condition.

This reverts part of commit 81fc303212b8379219cf5d463c8717359b972dba,
which probably didn't help portability anyway. Now, ebuild-ipc is using
non-blocking read with os.read and EAGAIN handling, just like
EbuildIpcDaemon since commit 7e5b81da12dd7bd59f6620840dc0d824e3f4d69a
(known compatible with FreeBSD).