vcl: improve vpp detached handling for mq epoll apps

vcl: improve vpp detached handling for mq epoll apps

Apps that rely on epoll and use eventfds for mq signaling can
potentially sleep indefinitely if vcl detaches from vpp and no libc fd
generates events.

To avoid this, at detach time, force creation of a pair of pipes that
constantly generates mq_epfd events to force apps, including ldp, to
request vppcom epoll_waits which retry attaching to vpp.

Type: improvement

Signed-off-by: Florin Coras <[email protected]>
Change-Id: Ie2ac338cc10721829e6ac525ee7d6b812354e9a3