major overhall of win32.c - not builds working rc with the mingw compiler.
respects $path and $pathext from environment (case insignificant); also
respects #! paths in scripts.

factored out the absolute path check into the OS specific files 
as this is different for win32 (drive letters).

try harder to discover the max filename component length for unix.

add makefiles for unix and win32

had to add -DPlan9 for native pla9 build, shame but less ugly than
nested if/then/else preprocessor clauses IMHO.

delay using the RCmain variable until after Vinit is called - on windows
we look for rcmain in our $path (lesser of many evils I feel). We do
this search in Vinit in OS specific code.

changes to haventfork.c to produce more helpful error messages
if an executable cannot be found (as plan9).

tweeks to error message formatting with line numbers for all OSs.

io.c:/^rewind/ had to change as it clashes with stdio, its not used anyway
so no problem.

-Steve