Implement server_address_new()

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.

src/nvim/eval.c

@@ -14572,7 +14572,7 @@ static void f_serverstart(typval_T *argvars, typval_T *rettv)
       rettv->vval.v_string = vim_strsave(get_tv_string(argvars));
     }
   } else {
-    rettv->vval.v_string = vim_tempname();
+    rettv->vval.v_string = (char_u *)server_address_new();
   }
 
   int result = server_start((char *) rettv->vval.v_string);

src/nvim/msgpack_rpc/server.c

@@ -2,6 +2,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <stdint.h>
+#include <inttypes.h>
 
 #include "nvim/msgpack_rpc/channel.h"
 #include "nvim/msgpack_rpc/server.h"
@@ -35,7 +36,7 @@ bool server_init(void)
   const char *listen_address = os_getenv(LISTEN_ADDRESS_ENV_VAR);
   if (listen_address == NULL) {
     must_free = true;
-    listen_address = (char *)vim_tempname();
+    listen_address = server_address_new();
   }
 
   bool ok = (server_start(listen_address) == 0);
@@ -67,6 +68,27 @@ void server_teardown(void)
   GA_DEEP_CLEAR(&watchers, SocketWatcher *, close_socket_watcher);
 }
 
+/// Generates unique address for local server.
+///
+/// In Windows this is a named pipe in the format
+///     \\.\pipe\nvim-<PID>-<COUNTER>.
+///
+/// For other systems it is a path returned by vim_tempname().
+///
+/// This function is NOT thread safe
+char *server_address_new(void)
+{
+#ifdef WIN32
+  static uint32_t count = 0;
+  char template[ADDRESS_MAX_SIZE];
+  snprintf(template, ADDRESS_MAX_SIZE,
+    "\\\\.\\pipe\\nvim-%" PRIu64 "-%" PRIu32, os_get_pid(), count++);
+  return xstrdup(template);
+#else
+  return (char *)vim_tempname();
+#endif
+}
+
 /// Starts listening for API calls on the TCP address or pipe path `endpoint`.
 /// The socket type is determined by parsing `endpoint`: If it's a valid IPv4
 /// address in 'ip[:port]' format, then it will be TCP socket. The port is