sockets: Add sysctl flag CTLFLAG_TUN to loader tunable

The sysctl variable 'kern.ipc.maxsockets' is actually a loader tunable.
Add sysctl flag CTLFLAG_TUN to it so that sysctl -T will report it
correctly.

No functional change intended.

Reviewed by: kib, imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42113