Restore cleaning of auto-generated kobj files.

[jhb]

Summary

Restore cleaning of auto-generated kobj files after the recent MFILES
changes. Use the list of MFILES found by find to identify the set of
possible auto-generated files and add the intersection of this set and
SRCS to CLEANFILES.

Test Plan

• make -V CLEANFILES in sys/modules/oce which has one of the files that the currently commented hack incorrectly removes

[imp]

This should nicely deal with the foo_if.c files that we have in the tree in modules builds that we have today.

[jhb]

Only pass __MPATH down to the modules to reduce duplication.

[sbruno]

Yep, this removes the leftover files when I try to build sys/modules/em when I do a make clean.

[jhb]

sys/conf/kern.pre.mk

231

Even simpler would be to pass the raw __MPATH down to sub-makes directly. Would then only do the make magic in one place (kmod.mk).