*: Rename main loop variable from loop to main_loop

Current name is inappropriate for the following reasons:

1. It is often masked by local `loop` variables.
2. It cannot be searched for. There are many `loop` variables where `loop` is
   some local variable. There are many cases when “loop” word is used in
   a comment.
3. It is in any case bad idea to use a generic name as a name of the global
   variable. Best if global has module prefix: this is why it is in `main.h`:
   `main_loop` both stands for “a main loop” and “a loop defined in `main.*`”.

Since I have no idea how to list every occurrence of this variable method used
to rename it is “remove it from globals.h, try to compile, fix errors”. Thus if
some occurrence was hidden under false `#if` branch it was not replaced.

src/nvim/msgpack_rpc/channel.c

@@ -16,6 +16,7 @@
 #include "nvim/event/socket.h"
 #include "nvim/msgpack_rpc/helpers.h"
 #include "nvim/vim.h"
+#include "nvim/main.h"
 #include "nvim/ascii.h"
 #include "nvim/memory.h"
 #include "nvim/os_unix.h"
@@ -119,7 +120,7 @@ void channel_teardown(void)
 uint64_t channel_from_process(char **argv)
 {
   Channel *channel = register_channel(kChannelTypeProc);
-  channel->data.process.uvproc = libuv_process_init(&loop, channel);
+  channel->data.process.uvproc = libuv_process_init(&main_loop, channel);
   Process *proc = &channel->data.process.uvproc.process;
   proc->argv = argv;
   proc->in = &channel->data.process.in;
@@ -127,7 +128,7 @@ uint64_t channel_from_process(char **argv)
   proc->err = &channel->data.process.err;
   proc->cb = process_exit;
   if (!process_spawn(proc)) {
-    loop_poll_events(&loop, 0);
+    loop_poll_events(&main_loop, 0);
     decref(channel);
     return 0;
   }
@@ -220,7 +221,7 @@ Object channel_send_call(uint64_t id,
   ChannelCallFrame frame = {request_id, false, false, NIL};
   kv_push(ChannelCallFrame *, channel->call_stack, &frame);
   channel->pending_requests++;
-  LOOP_PROCESS_EVENTS_UNTIL(&loop, channel->events, -1, frame.returned);
+  LOOP_PROCESS_EVENTS_UNTIL(&main_loop, channel->events, -1, frame.returned);
   (void)kv_pop(channel->call_stack);
   channel->pending_requests--;
 
@@ -316,11 +317,11 @@ void channel_from_stdio(void)
   Channel *channel = register_channel(kChannelTypeStdio);
   incref(channel);  // stdio channels are only closed on exit
   // read stream
-  rstream_init_fd(&loop, &channel->data.std.in, 0, CHANNEL_BUFFER_SIZE,
-      channel);
+  rstream_init_fd(&main_loop, &channel->data.std.in, 0, CHANNEL_BUFFER_SIZE,
+                  channel);
   rstream_start(&channel->data.std.in, parse_msgpack);
   // write stream
-  wstream_init_fd(&loop, &channel->data.std.out, 1, 0, NULL);
+  wstream_init_fd(&main_loop, &channel->data.std.out, 1, 0, NULL);
 }
 
 static void forward_stderr(Stream *stream, RBuffer *rbuf, size_t count,
@@ -647,7 +648,7 @@ static void close_channel(Channel *channel)
     case kChannelTypeStdio:
       stream_close(&channel->data.std.in, NULL);
       stream_close(&channel->data.std.out, NULL);
-      queue_put(loop.fast_events, exit_event, 1, channel);
+      queue_put(main_loop.fast_events, exit_event, 1, channel);
       return;
     default:
       abort();
@@ -692,7 +693,7 @@ static void close_cb(Stream *stream, void *data)
 static Channel *register_channel(ChannelType type)
 {
   Channel *rv = xmalloc(sizeof(Channel));
-  rv->events = queue_new_child(loop.events);
+  rv->events = queue_new_child(main_loop.events);
   rv->type = type;
   rv->refcount = 1;
   rv->closed = false;

src/nvim/msgpack_rpc/server.c

@@ -12,6 +12,7 @@
 #include "nvim/eval.h"
 #include "nvim/garray.h"
 #include "nvim/vim.h"
+#include "nvim/main.h"
 #include "nvim/memory.h"
 #include "nvim/log.h"
 #include "nvim/fileio.h"
@@ -108,7 +109,7 @@ int server_start(const char *endpoint)
   }
 
   SocketWatcher *watcher = xmalloc(sizeof(SocketWatcher));
-  socket_watcher_init(&loop, watcher, endpoint, NULL);
+  socket_watcher_init(&main_loop, watcher, endpoint, NULL);
 
   // Check if a watcher for the endpoint already exists
   for (int i = 0; i < watchers.ga_len; i++) {