bugs in FreeCOM

Hi!

1. FreeCOM incorrectly handles empty environment, _as this understanded by
   MS-DOS_. Trouble is that for empty environment MS-DOS places before
   16-bit counter not _one_, but _two_ zero bytes. This is easy to see: make
   empty config.sys with line INSTALL=FREECOM.COM and FreeCOM will not
   detect presence of path after environment.

   Solution (accept right variant and as MS-DOS):

/* env_seg != 0 */
const char FAR *p = MK_SEG_PTR (const char, env_seg);
while (*p)                              /* find end of environment */
  p += fstrlen (p) + 1;                 /* skip next variable */
if (FP_OFF (p) == 0)                    /* empty enviroment? */
{
  /* by definition empty enviornment should be
     "\0\1\0<path>\0", but MS-DOS make one more zero:
     "\0\0\1\0<path>\0"; accept both variants
  */
  if (*(const word FAR*)p == 0)         /* two zero bytes? */
    p++;                                /* skip one */
}

2. For empty config.sys with line SHELL=FREECOM.COM MS-DOS calls FreeCOM
   with PSP.ps_environ=0. This cause that FreeCOM not finds itself for
   strings resources. This is not bug, but unconvenient.

   Solution: before saying error message (when no environment, path not
   found after environment or path is wrong), FreeCOM should (like MS-DOS)
   try to find itself on boot drive (INT21/3305) with names like
   "\command.com", "\fdos\command.com", "\freedos\command.com" and
   "\dos\command.com". Also this search loop may be repeated with
   "freecom.com" name.

3. If FreeCOM doesn't finds own (startup) path (no environment or wrongly
   handled empty environment), then (with option /p) it just exits (with
   message, that path should be pointed as first argument) or (without /p)
   it asks to enter path (but should try to find itself, as in (2), then
   asks _name with path_). Also, after asking path, if FreeCOM anyway not
   finds itself (for example, command.com rename into other name), it again
   exits.

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