Keep right to bind on privileged ports under Linux

Only available on Linux systems with libcap.
Master then becomes cyrus before forking any child, and can (re)bind on
privileged ports upon SIGHUP.

Original code by Greg Banks, with necessary configure.ac section and code
macro to restrict its use to Linux systems (others may not have the same
features/functions/constants).
Recycle running services upon SIGHUP on master.

Master process now propagates SIGHUP to all running services, which triggers
their recycling.
Log processes that were not recycled within 30s

Done in janitoring code, so the test and log may be done a bit after 30s
No more need to loop over signals to handle

There is only one left, so as for the others, install it explicitly.
Make sure to catch more than one SIGHUP in services

Otherwise the second one really kills us
Refactor unecessary code in signals_reset_sighup_handler
Refactored unnecessary code in signals_add_handlers
signals: memset the struct sigaction

just to be entirely sure we're not feeding random bits to the kernel.
master: spawn undermanned services early

Otherwise interrupting signals can prevent master from reaching the desired
number of workers in a timely manner.