allow lxd to start w/o subuids

Per specs/userns-idmap.md, we should not fail to boot if there aren't enough
(or any) subuids, we should just not allow any operations that require
subuids.

This also re-enables the remote test.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Try to read remote pwd from stdin as a file

In case someone does 'echo foo | lxc remote add' (as we do in the tests), if we
get an ioctl error from trying to treat stdin as a tty, we should try to read
it as a file.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>