Ensure reconfig signal doesn't interrupt request dispatch when (!continuous_mode)

If SIGHUP is sent to a browser-switchboard process with continuous mode
off, the process will quit.  This is the right thing to do in most cases
(the next request will relaunch the process, at which point it'll read
the new config), but if we're in the middle of dispatching a request
when the signal arrives, quitting immediately will cause the request to
be lost.

Since browser-switchboard quits upon completion of a request when
continuous mode is off anyway, fix this by just ignoring SIGHUP while
handling requests when continuous mode is off.