RFC/RFT: split up pexecute.c

[DJ Delorie]

>           *-*-cygwin*        cygwin
>           *-*-mingw*         win32
>           *-*-winnt*         win32
>           *-*-msdosdjgpp*    djgpp
>           *-*-msdos*         msdos
> 
>           ???                os2
>           ???                mpw
> 
>           *                  unix  # incl. UWIN, VMS
> 
> The only OS/2 entry in config.guess is *-pc-os2-emx, which is
> described as a Unix compatibility layer, so I don't know which 
> version is appropriate for that.  There is no mention of MPW at all.

EMX defines OS2.  MPW is not currently supported by configure anyway,
so you can just leave that source file unused.  I believe the
remainder are correct.