index+xcpdb: improve SIG{INT,TERM,HUP,PIPE} behavior

index+xcpdb: improve SIG{INT,TERM,HUP,PIPE} behavior

-index now invokes ->DESTROY like xcpdb does, which is necessary
to cleanup $INBOX_DIR/msgmap-XXXXXXX files.  We'll also exit
with the expected values for various signals by adding 128
as described in <https://www.tldp.org/LDP/abs/html/exitcodes.html>

-xcpdb now terminates worker processes and xapian-compact(1)
invocations when prematurely killed, too.