In addition to the patch I posted yesterday, this one introduces
documentation for --disable-serial-configure and freinds in the output
of configure --help, and removes the nonopt argument from baseargs, so
that we don't pass --build=foo --host=foo foo down when the user runs
configure foo.  Christian Jönsson had run into this bug; this should
fix it.

Other than the fixes, this patch should not introduce any real changes
in behavior.  serial configure remains enabled by default.  I've
tested it with and without the options, and the generated serdep.tmp
files was as expected in all cases.

I'm checking this in.