Fix invocation of commands whose file name includes extension (Bug#19817)

Fix invocation of commands whose file name includes extension  (Bug#19817)

 nt/cmdproxy.c (get_next_token): Don't make backslashes disappear
 without a trace when they are not followed by a quote.
 (search_dir): Support searching programs whose file name already
 has an arbitrary extension.
 (main): When passing a command line to the shell, use cmd.exe
 rules for quoting command-line tail.