gocapability: upstream fix for unsporrted caps

@vmarmol has made the fix upstream for not failing if the capability
being dropped is returned as invalid from the syscall, which is the case
when the capability is not supported on the host.
This is a blocker presently for RHEL6.5 on CAP_SYSLOG. We have patched
around this in our RPM for the time being, but this is the proper fix.

See also https://github.com/dotcloud/docker/pull/5810

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #5869 from vbatts/vbatts-vendor-capabilities

gocapability: upstream fix for unsporrted caps