refactor(fs): now it is time to get rid of fs_loop and fs_loop_mutex

Here's the headline: when run in sync mode (last argument cb=NULL),
these functions don't actually use the uv_loop_t.

An earlier version of this patch instead replaced fs_loop with
using main_loop.uv on the main thread and luv_loop() on luv worker
threads. However this made the code more complicated for no reason.

Also arbitrarily, half of these functions would attempt to handle
UV_ENOMEM by try_to_free_memory(). This would mostly happen
on windows because it needs to allocate a converted WCHAR buffer.
This should be a quite rare situation. Your system is pretty
much hosed already if you cannot allocate like 50 WCHAR:s.
Therefore, take the liberty of simply removing this fallback.

In addition, we tried to "recover" from ENOMEM in read()/readv()
this way which doesn't make any sense. The read buffer(s) are already
allocated at this point.

This would also be an issue when using these functions on a worker
thread, as try_to_free_memory() is not thread-safe. Currently
os_file_is_readable() and os_is_dir() is used by worker threads
(as part of nvim__get_runtime(), to implement require from 'rtp' in
threads).

In the end, these changes makes _all_ os/fs.c functions thread-safe,
and we thus don't need to document and maintain a thread-safe subset.

src/nvim/main.c

@@ -266,10 +266,6 @@ int main(int argc, char **argv)
   // `argc` and `argv` are also copied, so that they can be changed.
   init_params(&params, argc, argv);
 
-  // Since os_open is called during the init_startuptime, we need to call
-  // fs_init before it.
-  fs_init();
-
   init_startuptime(&params);
 
   // Need to find "--clean" before actually parsing arguments.
@@ -1479,7 +1475,7 @@ static void init_startuptime(mparm_T *paramp)
 {
   for (int i = 1; i < paramp->argc - 1; i++) {
     if (STRICMP(paramp->argv[i], "--startuptime") == 0) {
-      time_fd = os_fopen(paramp->argv[i + 1], "a");
+      time_fd = fopen(paramp->argv[i + 1], "a");
       time_start("--- NVIM STARTING ---");
       break;
     }