The new{g,u}idmap binaries where a source of trouble for users when they lacked
sufficient privileges. This commit adds code to check for sufficient privilege.
It checks whether new{g,u}idmap is root owned and has the setuid bit set and if
it doesn't it checks whether new{g,u}idmap is root owned and has CAP_SETUID in
its CAP_PERMITTED and CAP_EFFECTIVE set.

Closes #296.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>