init: implement a new 'log' command, useful to debug property events

liblog is not usable, because stderr and stdout are redirected to
/dev/null to control the android log output.

Only forked processes are able to write to the android logs...

Note: accept parameters: log [-t topic] [-p level] <message>
      limit : strings beginning with "--" are not handled

Change-Id: I2fbf7422e0085d4cdf1bba6511413d1f796d4555