udp_send: Add/remove the udp socket fd only once.

udp_send: Add/remove the udp socket fd only once.

Currently, we remove this fd from the close-on-fork list in
udp_close_target() and re-add it in udp_init_session(). This is wrong
because we want the fd being closed in the command handlers also if
the child process for the command is spawned while the udp sender
is inactive.

So add/delete the fd when the udp target is added/removed rather than
on a per session basis.