mount_nfs: Add a "bgnow" NFS mount option

The "bg" option does not go background until the initial mount
attempt fails, which can take 60+ seconds.

This new "bgnow" option goes background immediately, avoiding
the 60+ second delay, if the NFS server is not yet available.

The man page update is a content change.

PR: 260764

(cherry picked from commit 08a8d16cbb269d4c2be02eb67351c59d16f4ecb9)