When converting a msgpack object to a String object, strings (and byte
arrays) with length 0 are handled as errors. This is fixed by
always using the msgpack data pointer as a valid pointer. For a NULL
pointer there is nothing to copy.

Test by @snoe

Fixes #3844

Return 1 if the endpoint argument is NULL, server_start() can get
a NULL value when using server_address_new() or vim_tempname(). Removed
the function attribute.

When creating a local socket/pipe (server_start()) Neovim used vim_tempname() to
generate a unique socket path. For Windows UNIX filepaths cannot be used as
pipe names (they must start with \\.\pipe\). This commit replaces the use of
vim_tempname() for server addresses with server_address_new().

server_address_new() generates unique names for local sockets/pipes - for UNIX
it uses vim_tempname(), for Windows generates names in the form

    \\.\pipe\nvim-PID-COUNTER

where PID is the current process id, and COUNTER is a static uint32_t counter
incremented with every call. This function is now used for server_start() and
server_init() when no address is available.

The call to `event_init()` was too late. `command_line_scan()` in `main()` could
already need the loop initialized. Ref https://github.com/neovim/neovim/issues/3045#issuecomment-123405833.

A consequence of this change is that it was necessary to move the
`channel_from_stdio()` call to `command_line_scan()` when embedded_mode is set.

- Explicitly set the SignalWatcher event queue. Without this, the watcher will
  publish events to the fast queue, resulting in resize bugs for certain
  terminals(#2322).
- Set `async = false` to the `remote_ui_attach` handler(It was a deferred
  before, this is the new equivalent)

This adds a redraw notification "update_menu" which is sent whenever
Vim's menus are changed by the :menu command and friends.

This is necessary to keep events in the same order received from the OS.