Do the debuglevel check before calling the print
function for _threaddebug, by making it a macro.
Do not waste cycles passing arguments.
Generalize the _threaddebug function into _threadprint()
and add a varargcheck pragma. This function can
also be used from _threadassert().
Fix missing arguments in one case, fix trailing
newlines in _threaddebug().
Make _threadgetproc()/_threadsetproc() a macro,
just dereferencing Proc**_threadprocp.
Simplify the mainjump, just call _threadsetproc()
directly without that mainp dance. Remove the
_schedinit() argument, it uses _threadgetproc() now.
Get rid of Mainarg struct, just have a global variable
for argc.
