fd: Add sysctl flag CTLFLAG_TUN to loader tunables

The following sysctl variables are actually loader tunables. Add sysctl
flag CTLFLAG_TUN to them so that sysctl -T will report them correctly.

1. kern.maxfiles
2. kern.maxfilesperproc

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 2af5ce5b5efb0cf6514acdc4a8b4058221641c39)
(cherry picked from commit d9aa723347825418c742164c3e9a77d1e4b6943d)