Remove --disable-userspace "configure" option, since it breaks "make dist".

Remove --disable-userspace "configure" option, since it breaks "make dist".

I had thought that Automake was smart enough to ignore conditionals around
EXTRA_DIST, so that all files always got distributed regardless of whether
Automake conditionals were set.  I was wrong.

This commit removes the --disable-userspace option to "configure", which
put a conditional around most of Makefile.am and thus unintentionally
caused most of the distribution to be left out if --disable-userspace was
specified.  The alternative (fixing --disable-userspace) seems like too
much work--it would require pushing "if ENABLE_USERSPACE" down into lots
of subdirectory--and would be difficult to maintain.