Hide 'Image Size' from install screen when installing in /data

fixes #115 https://github.com/guardianproject/lildebi/issues/115
calculating free size of internal storage from '/data' and not '/data/debian'

When lildebi is installed first time on a device, then /data/debian folder is
not present. So when the install screen (of installing in internal storage)
tries to calculate the free size of internal storage from /data/debian then an
exception is generated as /data/debian is not present.  So instead of
calculating free size from /data/debian, we will now calculate it from /data

fixes #114 https://github.com/guardianproject/lildebi/issues/114
use canonical/absolute path when finding free bytes on install path

The SD Card aka "external storage" on Android can do weird stuff with the
path to support the multi-user stuff.  Using the canonical path seems to
help with some of the issues, and if that fails, then try the absolute path.
Debian's /etc/profile needs real `id -u`, so use busybox's

Android's `id` is in the PATH when this little snipped of code is run in
Debian.  So when /etc/profile is first executed and is setting the PATH,
it needs `id -u` to return only a numeric value.

fixes #88 https://github.com/guardianproject/lildebi/issues/88
fixes #116 https://github.com/guardianproject/lildebi/issues/116
always show "Open Terminal"; guide user to install it if its not already

Instead of hiding "Open Terminal" from the menu when Terminal Emulator is
not installed, instead guide the user to install it if they click that
menu item.