Made booting with -a work for all configurations. Previously it

Made booting with -a work for all configurations.  Previously it
only worked for configurations with "swap on generic".

usr.sbin/config/config.y:
- ignore all "swap [on] device ...' specifications except for
  warning about them.  They haven't done anything related to swap
  for almost 4 years, and were previously silently ignored,
  except for "swap on generic" which stopped swap${KERNEL}.c
  from being generated.  Code to support swapping is now deader
  than before.

usr.sbin/config/mkswapconf.c:
- don't generate a dummy setconf() function in swap${KERNEL}.c.

sys/i386/conf/files.i386:
- swapgeneric.c is now standard.  It should be merged into autoconf.c
  so that it doesn't conflict with swap${KERNEL}.c for kernels named
  "generic".

sys/i386/i386/autoconf.c:
- don't call setroot() for mfs roots.  Since setroot() doesn't do anything
  harmful, this was just a waste of time, except possibly for booting with
  -a it may have helped prevent an undesireable call to setconf() by
  finding a bogus rootdev.
- honor -a for ffs roots.  -a now overrides all other ways of specifying
  the root device.  Previously, -r had precedence over -a, and the -a
  handling was usually a no-op.
- don't honor -a for non-ffs roots, since it would currently just get in
  the way of a clean panic.

sys/i386/i386/swapgeneric.c:
- don't declare things that are now always declared in swap${KERNEL}.c.
  Don't decide things that are now decided in autoconf.c.  Code to
  support the "generic" case is now dead instead of useless.