vlib: prevent some signals from being executed on workers

vlib: prevent some signals from being executed on workers

Before this commit, SIGINT, SIGHUP and SIGTERM could be executed on the
workers. Since those signals don't stop execution, it meant that atexit
handlers (like the `vl_unmap_shmem`) could run while the main thread was
still running, which can cause race conditions. To avoid that, we
prevent workers from handling those signals.

Type: fix
Signed-off-by: Guillaume Solignac <gsoligna@cisco.com>
Change-Id: I27a87d96a027d7423ced881a614427af4ab0f969