Fixed searching of $PATH in execvp(). Do what sh(1) should do according

Fixed searching of $PATH in execvp().  Do what sh(1) should do according
to POSIX.2.  In particular:

- don't retry for ETXTBSY.  This matches what sh(1) does.  The retry code
  was broken anyway.  It only slept for several seconds for the first few
  retries.  Then it retried without sleeping.
- don't abort the search for errors related to the path prefix, in
  particular for ENAMETOOLONG, ENOTDIR, ELOOP.  This fixes PR1487.  sh(1)
  gets this wrong in the opposite direction by never aborting the search.
- don't confuse EACCES for errors related to the path prefix with EACCES
  for errors related to the file.  sh(1) gets this wrong.
- don't return a stale errno when the search terminates normally without
  finding anything.  The errno for the last unsuccessful execve() was
  usually returned.  This gave too much precedence to pathologies in the
  last component of $PATH.  This bug is irrelevant for sh(1).

The implementation still uses the optimization/race-inhibitor of trying
to execve() things first.  POSIX.2 seems to require looking at file
permissions using stat().  We now use stat() after execve() if execve()
fails with an ambiguous error.  Trying execve() first may actually be a
pessimization, since failing execve()s are fundamentally a little slower
than stat(), and are significantly slower when a file is found but has
unsuitable permissions or points to an unsuitable interpreter.

PR:		1487