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