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
