Remove system makefile path directives from env passed to PORTS_MODULES step

Previously, MAKESYSPATH as well as '-m' directives in MAKEFLAGS would cause
any port rebuilt during the PORTS_MODULES stage to consume system makefiles
from $(SRCROOT)/share/mk instead of those installed under /usr/share/mk.
For kernel modules that need to build against an updated src tree this
makes sense; less so for <bsd.port.mk> or  any userspace library or utility
the port may also happen to install.

Before 11.0, this probably didn't matter much in practice.  But the addition
of src.libnames.mk under $(SRCROOT)/share/mk in 11.0 breaks any consumer of
bsd.prog.mk and DPADD/LDADD during PORTS_MODULES.

Address the build breakage by removing MAKESYSPATH and any occurrence of
'-m' from MAKEFLAGS in the environment created for the port build.
Instead set SYSDIR so that any kmod built by the port will still consume
conf/kmod.mk from the updated src tree, assuming it uses <bsd.kmod.mk>

Reviewed by:	bdrewery
MFC after:	2 weeks
Differential Revision: 	https://reviews.freebsd.org/D13053

sys/conf/kern.post.mk

@@ -69,6 +69,9 @@ PORTSMODULESENV=\
 	-u CC \
 	-u CXX \
 	-u CPP \
+	-u MAKESYSPATH \
+	MAKEFLAGS="${MAKEFLAGS:M*:tW:S/^-m /-m_/g:S/ -m / -m_/g:tw:N-m_*}" \
+	SYSDIR=${SYSDIR} \
 	PATH=${PATH}:${LOCALBASE}/bin:${LOCALBASE}/sbin \
 	SRC_BASE=${SRC_BASE} \
 	OSVERSION=${OSRELDATE} \