Change if_slowtimo_ch to a pointer

One benefit to do so is to reduce memory used for struct callout;
we can avoid to allocate struct callout for interfaces that don't
use callout.

Requested by uebayasi@.
Tweak ifconf variants

The tweaks make the code intention clear and make further changes easy.

No functional change.
fix comma error
Call looutput with holding KERNEL_LOCK

This fixes diagnostic assertion "KERNEL_LOCKED_P()" in if_loop.c.

PR kern/49410
Do not follow symlinks in sys_unmount()

There are situations where the underlying filesystem is unreachable
(e.g: NFS) causing symlink resolution to hang. Such a situation
should be avoided by using umount -f -R (force and raw), but while -R
causes the symlink resolution to be skipped in umount(8), the kernel was
still doing it in sys_unmount(). This changes fixes that.

When the -R flag is not given, umount(8) does symlinks resolution through
realpath(3) before calling unmount(2), hence not doing it in the kernel
would not change behavior.