CHROMIUM: eCryptfs: allow userspace messaging to be disabled

CHROMIUM: eCryptfs: allow userspace messaging to be disabled

When the userspace messaging (needed for ecryptfsd) is not needed, allow
eCryptfs to build with it removed. This saves on kernel code size and
reduces potential attack surface by removing the /dev/ecryptfs node.

BUG=chromium-os:35732
TEST=link build, /dev/ecryptfs is gone, eCryptfs still works.

[sent upstream for review:
 http://marc.info/?l=ecryptfs-users&m=136204079607329&w=2]

Change-Id: I1b2be61e0a455fc9f6e09a9f4a1b1f6d65d1903a
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/44278
Reviewed-by: Will Drewry <wad@chromium.org>