ash: further fixes to merge of upstream changesFRP-6075-g169694ebd

Commits e23652908 and 686a0803f (ash: fix execution of applets via
Unix-style path) were necessary following a major revision of the
shell upstream.

Another problem was found:

   $ sh
   $ PATH="/usr/bin;$PATH" exec non-existent

resulted in a segfault.  This happened because during a PATH search
tryexec() modified argv[0] when it detected a Unix-style path (in
this case '/usr/bin/non-existent') but failed to restore it if the
execution failed.

There were other issues:

- The logic of the new code failed to match the original:  the test
  for a Unix-style path should only have happened if an attempt to
  execute the full path had already failed.

- The test for a script running an interpreter which is an applet
  also modified argv.  If the execution failed argv should have
  been restored to its original state.

- During a PATH search the 'path' pointer walked through the
  elements of the path.  This pointer was also used to determine
  if an applet was overridden by an executable.  This is wrong:
  the full PATH variable should have been used.

The code around tryexec()/shellexec() has been rewritten to take
these issues into account.

Adds 32-48 bytes.

(GitHub issue #584)

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