logd: consolidate command handling and fix an error check

Consolidate command handling for commands that take a buffer ID as
their first argument.  Further use ParseUint instead of atoi(), since
atoi("bad string") returns `0` is the same as LOG_ID_MAIN.

Test: these commands are handled property
Test: `echo -n 'getLogSize ignoreme!\0' | nc -U /dev/socket/logd`
      returns an error
Change-Id: I3e84a6157e16f4c110681da453e1ae1a920da7c4