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/runtime.c

@@ -1559,7 +1559,7 @@ static inline char *add_env_sep_dirs(char *dest, const char *const val, const ch
     return dest;
   }
   const void *iter = NULL;
-  const char *appname = get_appname();
+  const char *appname = get_appname(false);
   const size_t appname_len = strlen(appname);
   do {
     size_t dir_len;
@@ -1626,7 +1626,7 @@ static inline char *add_dir(char *dest, const char *const dir, const size_t dir_
     if (!after_pathsep(dest - 1, dest)) {
       *dest++ = PATHSEP;
     }
-    const char *appname = get_appname();
+    const char *appname = get_appname(false);
     size_t appname_len = strlen(appname);
     assert(appname_len < (IOSIZE - sizeof("-data")));
     xmemcpyz(IObuff, appname, appname_len);
@@ -1697,7 +1697,7 @@ char *runtimepath_default(bool clean_arg)
   size_t config_len = 0;
   size_t vimruntime_len = 0;
   size_t libdir_len = 0;
-  const char *appname = get_appname();
+  const char *appname = get_appname(false);
   size_t appname_len = strlen(appname);
   if (data_home != NULL) {
     data_len = strlen(data_home);