Add 'run-as' command implementation as set-uid program.

Typical usage is 'run-as <package-name> <command>' to run <command>
in the data directory, and the user id, of <package-name> if, and only
if <package-name> is the name of an installed and debuggable application.

This relies on the /data/system/packages.list file generated by the
PackageManager service.

BEWARE: This is intended to be available on production devices !