Fix SIGHUP handling by unconnected bgworkers

Add a SignalUnconnectedWorkers() call so that non-database-connected background
workers are also notified when postmaster is SIGHUPped.  Previously, only
database-connected workers were.

Michael Paquier (bug report and fix)
Fix SIGUSR1 handling by unconnected bgworkers

Latch activity was not being detected by non-database-connected workers; the
SIGUSR1 signal handler which is normally in charge of that was set to SIG_IGN.
Create a simple handler to call latch_sigusr1_handler instead.

Robert Haas (bug report and suggested fix)