This single commit performs two major actions:
1. Reverts all instances of the 'irssip' custom branding back to the original 'irssi' naming. This includes a comprehensive and precise update of the binary name, default home directory, configuration files, documentation, tests, and all internal #include paths. The 'irssiproxy' module name is intentionally left unchanged.
2. Introduces 'configure.sh', a portable shell script that guides the user through the installation process, offering side-by-side installation to prevent conflicts.
This approach ensures a clean, atomic change that prepares the branch for both upstream pull requests and user-friendly installation.
