We get enable_irq()/disable_irq()

The C++ experiment is not going well.  Get rid of the 'extern "C"', but
replace it with an "asmlinkage" #define that allows us to experiment.

ELF binary support it a notable change.

Original ChangeLog:

 - the bad memory management one-liner bug in pl12 is naturally fixed.
 - compiled with plain C by default instead of C++
 - ELF binary support (Eric Youngdale)
 - Quickport mouse support (and some changes to the PS/2 mouse driver)
   by Johan Myreen and co)
 - core file name change ("core" -> "core.xxxx" where xxxx is the name
   of the program that dumped code).  Idea from ???.  Also, core-files
   now correctly truncate any existing core file before being written.
 - some mmap() fixes: better error returns, and handling of non-fixed
   maps for /dev/mem etc.
 - one kludgy way to fix the wrong arp packets that have plagued net-2d
   (resulting in arp packets that had the first four bytes of the
   ethernet address as the IP address).
 - I fixed the mount-point handling of 'rename()' and 'unlink()/rmdir()'
   so that they should now work and/or give appropriate error messages.
   An early version of this patch was already sent to the KERNEL
   channel, which fixed the rename problem but not a similar bug with
   unlink.
 - packet mode fixes by Charles Hedrick.  Sadly, these are likely to
   break old telnet/rlogin binaries, but it had to be done in order to
   communicate correctly with the rest of the world.
 - FPU emulator patches from Bill Metzenthen.  The fprem1 insn should be
   correct now (not that anybody seems to have seen the incorrect
   behaviour..)
 - a few fixes for SCSI (Drew and Eric)
 - signal.c changes to handle multiple segments (for Wine) correctly.
 - updated drivers from Donald Becker: 3c509 and AT1500 drivers, but
   also some other drivers have been edited, and some networking fixes.