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

src/nvim/eval/funcs.c

@@ -7641,7 +7641,7 @@ static void get_xdg_var_list(const XDGVarType xdg, typval_T *rettv)
     return;
   }
   const void *iter = NULL;
-  const char *appname = get_appname();
+  const char *appname = get_appname(false);
   do {
     size_t dir_len;
     const char *dir;