Install Abseil failure signal handler in distributor/proton daemons

This will attempt to dump a stack trace for the offending thread
to stderr, which greatly improves visibility for everyone running
Vespa on systems with core dumps disabled.

Signal handler chaining is explicitly enabled to allow sanitizer
handlers to be called as expected.

Note that we install our own signal handlers _after_ the Abseil
handlers to avoid noisy stack dumping on `SIGTERM`. It is considered
a fatal signal by the failure handler, but the config sentinel
uses it as a friendly "please shutdown now, or else" nudge in the
common case.

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