create_run_template: tell the template what caller's uid was mapped to
conf.c/conf.h: have replaced bool hostid_is_mapped() with int mapped_hostid()
which returns the mapped uid for the caller's uid on the host, or -1 if
none
create_run_template: pass caller's uid into template.
lxc-ubuntu-cloud:
1. accept --mapped-uid argument
2. don't write to devices cgroup - not allowed.
3. if running in userns, use $HOME/.cache
4. chown cached files to the uid to which our caller was
mapped
5. ignore /dev when extracting rootfs in a userns