fix(startup): server fails if $NVIM_APPNAME is relative dir #30310

Problem:
If $NVIM_APPNAME is a relative dir path, Nvim fails to start its
primary/default server, and `v:servername` is empty.
Root cause is d34c64e342, but this wasn't
noticed until 96128a5076 started reporting the error more loudly.

Solution:
- `server_address_new`: replace slashes "/" in the appname before using
  it as a servername.
- `vim_mktempdir`: always prefer the system-wide top-level "nvim.user/"
  directory. That isn't intended to be specific to NVIM_APPNAME; rather,
  each *subdirectory* ("nvim.user/xxx") is owned by each Nvim instance.
  Nvim "apps" can be identified by the server socket(s) stored in those
  per-Nvim subdirs.

fix #30256

test/functional/testnvim.lua

@@ -14,8 +14,7 @@ local is_os = t.is_os
 local ok = t.ok
 local sleep = uv.sleep
 
---- This module uses functions from the context of the test session, i.e. in the context of the
---- nvim being tests.
+--- Functions executing in the current nvim session/process being tested.
 local M = {}
 
 local runtime_set = 'set runtimepath^=./build/lib/nvim/'