Fix kern/187712: config(8) does not respect KERNCONFDIR.
The impact of this bug is that you cannot build a kernel if both of the
following are true:

1. The kernel config file is in a non-default location
2. The kernel config file uses the "include" statement from config(5).

usr.sbin/config/main.c
usr.sbin/config/config.8
usr.sbin/config/config.h
usr.sbin/config/lang.l
Added a "-I path" option to config(8). By analogy to cc(1), it adds
an extra path in which the "include" statement will search for
files.

Makefile.inc1
Pass "-I ${KERNCONFDIR}" to config(8).

PR: kern/187712
Reviewed by: will, imp (previous version)
MFC after: 3 weeks
Sponsored by: Spectra Logic Corporation