Cleanup some GG utility functions

* Remove unused `ggp_int64_compare`.
* Inline `ggp_list_copy_to_slist_deep`.
It's only used once.
* Rewrite `ggp_message_format_to_gg` using `GString`.
This may not be the best, but it's better than the implementation with `GList`.
* Use a string list pref to save gg server history.
This removes the need for gg to do the conversion to/from plain strings itself,
and thus removes the need for the `ggp_str*_list` functions.
* Embed `ggp_list_truncate` directly where used, and in simplified form.
* Embed `ggp_password_validate` at its caller.
There's only one use of it.

Testing Done:
Compile only.

Reviewed at https://reviews.imfreedom.org/r/117/