Low: libcrmcommon: Standardize pcmk_poll_ipc() return code

From poll(2) man page:

On some other UNIX systems, poll() can fail with the error EAGAIN if the
system fails to allocate kernel-internal resources, rather than ENOMEM
as Linux does. POSIX permits this behavior.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>