logcatd: introduce logcatd executable

logcatd is the same as logcat, except that the -L flag, if supplied,
runs once, then the command re-runs itself without the -L flag with
the same argument set.  By introducing a logcatd daemon executable
we can solve the problem of the longish reads from pstore that
sometimes occur when the system is excessively busy, starving this
background cgroup daemon as we absorb the delay in a backgrounded
init "service", rather than in a forgrounded init "exec".  This would
not have been efficiently possible without the introduction of

There are no doubt many flags that make no sense to run twice with,
and without, the -L flag.  In the general sense we expect the caller
to perform the correct set of operations and not pick these nonsense
operations.  logcatd is only supplied on engineering and debug builds
for logpersist, and is only an automated aid to triage.

Test: gTest logcat-unit-tests
Test: manual confirm logpersist functions as expected, required reboot
Bug: 28788401
Bug: 30041146
Bug: 30612424
Bug: 35326290
Change-Id: I53ba31970749daf37eef42636f039f485932416f
6 files changed