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 spinning in a
foreground task starving this daemon as we absorb the delay in
an init service, rather than in an init exec.  This would not have
been efficiently possible without the introduction of liblogcat.

Test: gTest logcat-unit-tests
Test: Manual check logpersist operations
Bug: 28788401
Bug: 30041146
Bug: 30612424
Bug: 35326290
Change-Id: I3454bad666c66663f59ae03bcd72e0fe8426bb0a

private/file_contexts

@@ -172,6 +172,7 @@
 /system/bin/toolbox	--	u:object_r:toolbox_exec:s0
 /system/bin/toybox	--	u:object_r:toolbox_exec:s0
 /system/bin/logcat	--	u:object_r:logcat_exec:s0
+/system/bin/logcatd	--	u:object_r:logcat_exec:s0
 /system/bin/sh		--	u:object_r:shell_exec:s0
 /system/bin/run-as	--	u:object_r:runas_exec:s0
 /system/bin/bootanimation u:object_r:bootanim_exec:s0