select: add functionality to check socket state

osmo_fd_register() is used to register socket file descriptors,
after registering a socket, there is no way to test if the socket
is still registered or actually registered at all.

This commit adds a new function osmo_fd_register_check() that can
be used to check in advance, if the socket fd is registered,
before performing further operations.

Change-Id: I48ec7098d6bba586c81bf0d5c9088108e2c081c6
2 files changed