Coding style: Get rid of the low-hanging fruits

This patch takes care of the following:

- Prefer `g_assert()` over `assert()`.
- Prefer explicit NULL checks: `if (foo == NULL)` over `if (!foo)`.
- Rename the `SERVER_REC *irssi` variable to `SERVER_REC *server`.
- Kill `get_client_config_dir()`, `IRSSI_NICK` and `IRSSI_CONN_ADDR` macros.
- Eliminate `goto`-based error handling.