win: jobstart(), system(): $PATHEXT-resolve exe

Windows: In order for jobstart(['foo']), system(['foo']) to find
"foo.cmd", we must replace "foo" with "foo.cmd" before sending `argv` to
process_spawn().

Rationale: jobstart([…]), system([…]) "executable" semantics should be
consistent with the VimL executable() function.

fix #9569
related: #10554

test/functional/fixtures/cmdscript.cmd

@@ -0,0 +1,2 @@
+@echo off
+echo much success