The documentation of namespace callbacks was inconsistent on whether
it preserved positive return values.  Now that we have a dedicated
EXIT_CANCELED to flag all errors before getting to the callback,
it is possible to use positive return values (not that any of the
current callers do, but it is better to match the docs).

Also, while vircommand.c is careful to close fds that a child should
not have, it's still better to be in the practice of setting
FD_CLOEXEC up front.

* src/util/virprocess.c (virProcessRunInMountNamespace): Tweak
return value to pass back non-zero status.  Avoid leaking pipe fds
to other threads.
* src/util/virprocess.h: Fix comment.
Signed-off-by: NEric Blake <eblake@redhat.com>