ash: changes to command resolution, execution and display

ash separates searching for commands from running them.  Searching
is performed in find_command().  The result of the search is stored
in a hash table but not as the full path:  the path is reconstructed
in shellexec() by combining the directory and command name.

In Windows the command name is insufficient, as the executable name
may also include an extension.  To resolve this we must ensure that
extensions are processed in the same order in find_command, shellexec
and any other places where command names are resolved into file names.

The order used matches that of spawnve:  .com, .exe, .bat, .cmd.
Finally the bare filename with no additional extension

The order has been made consistent:

- in find_command for absolute paths and the Windows-specific path
  search;
- in tryexec (which is called by shellexec) where an additional test
  that the file is executable has been added;
- in the type and hash built-ins where the output has been modified to
  include the extension.

Additionally, the code in tryexec to handle ENOEXEC errors from
execve has been excluded.  The rationale for the exec functions
in POSIX [1] suggests the requirement to retry is a workaround
for the lack of '#!' support on some platforms.

[1] http://pubs.opengroup.org/onlinepubs/009695399/functions/exec.html

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