errno-util: add new ERRNO_IS_ACCEPT_AGAIN() test

errno-util: add new ERRNO_IS_ACCEPT_AGAIN() test

This is modelled after the existing ERRNO_IS_RESOURCES() and in
particular ERRNO_IS_DISCONNECT(). It returns true for all transient
network errors that should be handled like EAGAIN whenever we call
accept() or accept4(). This is per documentation in the accept(2) man
page that explicitly says to do so in the its "RETURN VALUE" section.

The error list we cover is a bit more comprehensive, and based on
existing code of ours. For example EINTR is included too (since we need
that to cover cases where we call accept()/accept4() on a blocking
socket), and of course ERRNO_IS_DISCONNECT() is a bit more comprehensive
than the list in the man page too.