Emacs populates 'exec-path' at launch from the 'PATH' environment
variable. Likewise, the emacs derivation from nixpkgs populates
'load-path' from the 'NIX_PROFILES' variable. As neither of these are
available by default in the systemd user manager, revert to the
previous behavior of launching the Emacs daemon from a login shell.
Fixes#1354Fixes#1340
PR #1355