vlib: exit 0 (nocore) on SIGHUP

Scenarios where SIGHUP is sent would include the user closing an xterm
while in interactive/nodaemon mode, or similarly when running vpp in the
same mode during testing (e.g., using ssh to run VPP on a DUT). VPP
should exit in these cases; however, generating a core is unwanted.

Type: fix
Signed-off-by: Christian Hopps <chopps@labn.net>
Change-Id: Ibccfe5e676547e913c8a205ff16ab56d9abb1c82