LP1204123 SIGUSR2 causes router re-register (Perl/C)

LP1204123 SIGUSR2 causes router re-register (Perl/C)

When a listener process receives the USR2 signal, the 'register' command
is sent to all known routers.  This allows for the addition of new
routers to running opensrf services.

If a register command is sent to a router for which the process is
already successfully register, the router will safely discard it, so
multiple SIGUSR2 calls are safe.

Minor int => bool clarification repair to existing C shutdown
(prefork_clear) code.

Signed-off-by: Bill Erickson <berick@esilibrary.com>