amigaos4: move the amigaos exec code under amigaos4

amigaos4: move the amigaos exec code under amigaos4

Largely reimplements 839a9f02, 54fa14d7, e8432c63, 40262ff4.

The upside is that now doio.c and pp_sys.c have much less AmigaOS
specific ifdefs. As a downside, the exec code is now forked (pun
only partially accidental.)

The earlier story regarding fork+exec, that the AmigaOS creating
thread doesn't terminate but instead continues running is both true
and false.  The more detailed story is that the user-observable
behaviour is as with POSIX/UNIX.  The thread that created the new
"task" (to use the AmigaOS terms) does hang around -- but all it
does is to wait for the new task to terminate, and more importantly,
it holds on to the resources like filehandles.  If the task were to
immediately terminate, the resources would be reclaimed by the kernel.