When a machine boots the NFS mounting script is executed after
interfaces are configured, but for many interfaces (e.g. all Intel)
ifconfig causes link renegotiation, so the first attempt to mount
NFS always fails. After that mount_nfs sleeps for 30 seconds, while
only a couple seconds are actually required for interface to get up.

Instead of sleeping, do select(2) on routing socket and check if
some interface became UP and in this case retry immediately.

Reviewed by: rmacklem
Differential Revision: https://reviews.freebsd.org/D23934