uidmap: fix writing multiple ranges

uidmap: fix writing multiple ranges

The kernel requires a single atomic write for setting the /proc
idmap files. We were calling write(2) more than once when multiple
ranges were configured so instead build a buffer to pass in one write(2)
call.

Change id types to unsigned long to handle large id mappings gracefully.

Fix max id in example comment.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>