init.rc: logd: Add logpersistd (nee logcatd)

(cherry pick from commit 100658c3)

- logpersistd is defined as a thread or process in the context of the
  logd domain. Here we define logpersistd as logcat -f in logd domain
  and call it logcatd to represent its service mechanics.
- Use logcatd to manage content in /data/misc/logd/ directory.
- Only turn on for persist.logd.logpersistd = logcatd.
- Add logpersist.start, logpersist.stop and logpersist.cat debug
  class executables, thus only in the eng and userdebug builds.

ToDo: Wish to add Developer Options menu to turn this feature on or
off, complicated by the fact that user builds have no tools with
access rights to /data/misc/logd.

Bug: 19608716
Change-Id: I57ad757f121c473d04f9fabe9d4820a0eca06f31