xap_helper: implement alarm(2)-based timeout

xap_helper: implement alarm(2)-based timeout

alarm(2) delivering SIGALRM seems sufficient for Xapian since
Xapian doesn't block signals (which would necessitate the use of
SIGKILL via RLIMIT_CPU hard limit).  When Xapian gets stuck in
`D' state on slow storage, SIGKILL would not make a difference,
either (at least not on Linux).

Relying on RLIMIT_CPU is also trickier since we must account for
CPU time already consumed by a process for unrelated requests.
Thus we just rely on a simple alarm-based timeout.  This also
avoids requiring the optional BSD::Resource module in the (mostly)
Perl implementation (and avoids potential bugs given my meager
arithmetic skills).