jobstart(): Fix hang on non-executable cwd #9204

* os/fs.c: add os_isdir_executable()
* eval.c: fix hang on job start caused by non-executable cwd option
* channel.c: assert cwd is an executable directory
* test: jobstart() produces error when using non-executable cwd

src/nvim/channel.c

@@ -284,6 +284,8 @@ Channel *channel_job_start(char **argv, CallbackReader on_stdout,
                            uint16_t pty_width, uint16_t pty_height,
                            char *term_name, varnumber_T *status_out)
 {
+  assert(cwd == NULL || os_isdir_executable(cwd));
+
   Channel *chan = channel_alloc(kChannelStreamProc);
   chan->on_stdout = on_stdout;
   chan->on_stderr = on_stderr;