This replaces #7433. The improvements are:
1) Install the systemd user service in the proper directory depending
on if it's a 'user' install or a 'system' install. This is controlled
either by using the `--system` build flag (as most packages will) or by
the `-Dsystem-package` flag.
2) Add the absolute path to the `ghostty` binary in the application
file, the DBus service, and the systemd user service. This is done so
that they do not depend on `ghostty` being in the `PATH` of whatever
is launching Ghostty. That `PATH` is not necessarily the same as the
`PATH` in a user shell (especially for DBus activation and systemd user
services).
3) Adjust the DBus bus name that is expected by the system depending on
the optimization level that Ghostty is compiled with.
Reverts two commits:
977cd530c7bb9551de93900170bdaec4601b1b5b
820b7e432b57cd08c49d2e76cce4cb78016f0418
These break build from source on Linux for two reasons:
1.) The systemd user service needs to be installed in the `share`
prefix, not the `lib` prefix. This lets it get picked up in `~/.local`
but is also correct for just standard FHS paths.
2.) The `ghostty` path in the systemd user service needs to be absolute.
We should interpolate in the build install prefix to form an absolute
path.
