Thanks to everyone who replied; the XFree86 problem turned out to be two
header files in mit/server/ddx/x386/etc that need to be installed (I haven't
yet found a mention of it in the README files, but I'm sure I will eventually,
and then I'll feel like an idiot).  Inconveniently, doing "make install" in
/usr/src/include deletes the /usr/include/machine directory, meaning that
those two header files need to get reinstalled.  Grr.

The kernel build problem probably was an out-of-date makefile somewhere;
the kernel makefile assumes the object directory in sys/lib/libkern will
be named "obj.i386", but "make obj" there had created just "obj"; rerunning
"make obj" last night created the right name, and everything is happy again
(the makefile in sys/lib/libkern could cope with either name, it turns out,
but not the kernel makefile).

The ethernet problem turned out to be a configuration screwup -- I had
redone my config file from the latest ones and hadn't noticed the addition
of the CD ROM driver expecting its registers at 0x300;  the probe routine
for the CD ROM ran before the ethernet probe, and managed to really do a
number on the Ethernet card's configuration...

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