Because this provider only applies when the posix feature is present (and thus
not the windows feature), it can never be used on Windows. Thus, the
Windows-specific command handling is unnecessary and unused.

Also added more specific error messages for the cases where a command doesn't
exist, isn't a file, and isn't executable. These only apply when the command
path is absolute (otherwise the message is simply command not found).

Reviewed-By: Matt Robinson <matt@puppetlabs.com>

The documentation for the shell and posix providers didn't fully explain the
differences between them or the security implications of each. This commit
improves the documentation of both providers.

This makes it possible to use shell builtins when the exec is inline
bash commands.

Paired-with: Max Martin

This is in preparation for allowing other new providers to handle exec
commands differently.

Reviewed-by: Max Martin and Matt Robinson