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
Approved by: re (gjb)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42113

(cherry picked from commit 978be1ee5d5fef983b4163955e15c4269c0ec76d)
(cherry picked from commit e4f97a1ec798d69b8ebf767d0dd2e43fbb1550f6)