ifconfig(8): Don't set network interface capabilities when there is no change.

A quick grep through the kernel code shows network drivers compute the
changed bits of network capabilities after a SIOCSIFCAP IOCTL(2) by
using the bitwise exclusive or operation. When the set capabilities
are equal to the already read capabilities, no action will be taken.

Let ifconfig(8) predict this case and skip the SIOCSIFCAP IOCTL(2)
system call.

Discussed with: kib@ (revert change in case of issues)
MFC after: 1 week
Sponsored by: NVIDIA Networking