Some platforms provide an implementation of socket() and pipe2() that allow the
caller to specify that the resulting file descriptors should be non-blocking.

Using these allows us to potentially elide 3 calls into 1 by avoiding extraneous
calls to fcntl() to set the O_NONBLOCK flag afterwards.

In passing, change ast_alertpipe_init() to use pipe2() directly instead of the
wrapper if it is available.

Change-Id: I3ebe654fb549587537161506c6c950f4ab298bb0