win: libuv_process_spawn(): special-case cmd.exe

Disable CommandLineToArgvW-standard quoting for cmd.exe.

libuv assumes spawned processes follow the convention expected by
CommandLineToArgvW(). But cmd.exe is non-conformant, so for cmd.exe:
- With system([]), the caller has full control (and responsibility) to
  quote arguments correctly.
- With system(''), shell* options are used.

libuv quoting is disabled if argv[0] is:
- cmd.exe
- cmd
- $COMSPEC resolving to a path with filename cmd.exe

Closes #6329
References #6387

1 files changed, 12 insertions, 10 deletions

diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 1ed919a241..aa979ae42c 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -4852,16 +4852,18 @@ jobstart({cmd}[, {opts}])				{Nvim} *jobstart()*
 		Spawns {cmd} as a job.  If {cmd} is a |List| it is run
 		directly.  If {cmd} is a |String| it is processed like this: >
 		  :call jobstart(split(&shell) + split(&shellcmdflag) + ['{cmd}'])
-<		NOTE: This only shows the idea; see |shell-unquoting| before
-		constructing lists with 'shell' or 'shellcmdflag'.
-
-		NOTE: On Windows if {cmd} is a List, cmd[0] must be a valid
-		executable (.exe, .com).  If the executable is in $PATH it can
-		be called by name, with or without an extension: >
-		   :call jobstart(['ping', 'neovim.io'])
-<		If it is a path (not a name), it must include the extension: >
-		   :call jobstart(['System32\ping.exe', 'neovim.io'])
-<
+<		(Only shows the idea; see |shell-unquoting| for full details.)
+
+		NOTE: on Windows if {cmd} is a List:
+		  - cmd[0] must be executable. If it is in $PATH it can be
+		    called by name, with or without an extension: >
+		      :call jobstart(['ping', 'neovim.io'])
+<		    If it is a path (not a name), extension is required: >
+		      :call jobstart(['System32\ping.exe', 'neovim.io'])
+<		  - {cmd} is quoted per the convention expected by
+		    CommandLineToArgvW https://msdn.microsoft.com/bb776391
+		    unless the first argument is some form of "cmd.exe".
+
 		{opts} is a dictionary with these keys:
 		  on_stdout: stdout event handler (function name or |Funcref|)
 		  on_stderr: stderr event handler (function name or |Funcref|)