Handle <cword>, <cfile>, etc. properly in Syscall.CallForeground

[dbarnett]

Syscall.CallForeground and the s:EscapeSpecialChars function it uses don't have any special handling for <cword>, <cfile>, etc. Should they?

[dbarnett]

@Kashomon

I'm filing this to follow up on what was a TODO in the code, but I suspect we don't want to handle those values specially since I don't think CallForeground should support syntax that Call doesn't. I think it should just dumbly escape the special characters in them and pass them to the shell, and code can explicitly fetch cword and other values in vimscript if necessary.