logcat: modernize the code

1) Use libbase parsing functions instead of our own versions
2) Remove log_device_t, as it's an unneeded wrapper around log buffers
3) Do not report 'unexpected' if we get a log from a buffer that we
   didn't reqest.  I checked and I don't see any references to this
   happening in practice, so the code is mostly superfluous.  It seems
   reasonable enough to report the actual buffer name instead of
   'unexpected' in any case.
4) Print all buffers that experience an error, not just the first one
   seen.

Test: logcat works, logcat-unit-tests
Change-Id: Ic50074cfb716f63f3eda261f1cd236d7afaf453a