selinux: enable per-file labeling for functionfs

This patch adds support for genfscon per-file labeling of functionfs
files as well as support for userspace to apply labels after new
functionfs endpoints are created.

This allows for separate labels and therefore access control on a
per-endpoint basis. An example use case would be for the default
endpoint EP0 used as a restricted control endpoint, and additional
usb endpoints to be used by other more permissive domains.

It should be noted that if there are multiple functionfs mounts on a
system, genfs file labels will apply to all mounts, and therefore will not
likely be as useful as the userspace relabeling portion of this patch -
the addition to selinux_is_genfs_special_handling().

This patch introduces the functionfs_seclabel policycap to maintain
existing functionfs genfscon behavior unless explicitly enabled.

Signed-off-by: Neill Kapron <nkapron@google.com>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
[PM: trim changelog, apply boolean logic fixup]
Signed-off-by: Paul Moore <paul@paul-moore.com>