consolidate some arg processing

Unix and Windows had diverged significantly for command line
options handling.

This:
   1. uses the the Unix processing as a baseline.
   2. consolidates the code in earlyarg.c, where it can
      be a common copy to be shared.
   3. start converting the Windows command line argument
      processing to the Unix code that now resides in earlyarg.c.

sys/share/pcmain.c

@@ -671,7 +671,7 @@ nhusage(void)
 
 #ifdef CHDIR
 void
-chdirx(char *dir, boolean wr)
+chdirx(const char *dir, boolean wr)
 {
 #ifdef AMIGA
     static char thisdir[] = "";