Android support

This required modifying Linux.pm a bit, since Android is basically a barebones Linux, so both os_is('Unix') and os_is('Linux') need to return true for it.

Also, I should point out that currently the module installs cleanly on Android, which was very surprising; nothing seemed wrong until another module tried doing a os_is('Unix'), which returned false and sent me chasing down the dependency tree. Maybe the module should die at build time if it doesn't know the OS it's on?