win32: search PATH for missing Unix-style executables

Commit 41ef232fc5 (win32: use built-in applets for non-existent
binaries with Unix-style paths) alters what happens when trying
to find an executable.  If all of the following apply:

- the pathname starts with one of the standard directories for Unix
  executables (/bin, /usr/bin, /sbin, /usr/sbin);

- the file isn't found relative to the system root;

- the basename matches an applet

then the applet is run.

Further extend the procedure so that if the first two conditions are
met and either:

- the PREFER_APPLETS and SH_STANDALONE features are enabled and the
  basename *doesn't* match an applet

or

- the PREFER_APPLETS and SH_STANDALONE features are disabled

then PATH is searched for the basename.

This affects:

- how interpreters and binaries are spawned by mingw_spawn_interpreter()
  and mingw_spawnvp();

- how 'which' and the shell search for binaries.

Special steps need to be taken in the shell to avoid treating shell
built-ins and functions as applets.

As a consequence of this change:

- An executable that isn't an applet, say curl.exe, can be run as
  /usr/bin/curl so long as it's in a directory in PATH.  It doesn't
  have to be in C:/usr/bin.

- If the PREFER_APPLETS and SH_STANDALONE features are disabled binaries
  can be run using paths referring to standard Unix directories even if
  they're installed elsewhere in PATH.

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