/etc/rc.subr: add support for kld sysctl variables

For kernel modules loaded by scripts in /etc/rc.d and
/usr/local/etc/rc.d, if there is a file in /etc/sysctl.conf.d named <kld
name>.conf, then this will be loaded using the sysctl(8) utility. For
instance, sysctl variable changes for the pf kernel module would be
placed in the file /etc/sysctl.conf.d/pf.conf.

PR: 272129
Reviewed by: imp freebsd_igalic.co
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D40886