shell-quote-argument bug on w32

[Lennart Borgman]

shell-quote-argument actually is an "os quote argument". It does not 
check what shell is actually used. This is a bug in my opinion, but you 
notice it mostly on w32 where different shells are actually used.

I think therefore that shell-quote-argument should check shell-file-name 
at least when system-type is 'windows-nt. My suggestion for fixing this 
is introducing a variable like below and do the needed changes to 
`shell-quote-argument':

(defcustom shell-quote-types
 '( ("cmdproxy" w32cmd) )
 "Type of quoting to use for inferior shell.
A list of cons cells where the car of each cell is a regexp that
is matched against `shell-file-name' by
`shell-quote-argument'. When a match is found the corresponding
quoting type given by the cdr of the cons cell is used."
 :type '(repeat (list regexp symbol))
 :group 'shell)