Fix a few minor issues with `make distclean`.

- Only remove the base directory for in-source installs.
- Use full paths to the source directory.
- Update the help message to be more accurate.
Merge pull request #509 from SaberUK/insp20+make-distclean

Fix a few minor issues with `make distclean`.
Fix disabling IPV6_V6ONLY on sockets, issue #511
Fix logfile path on Windows

Fixes issue #510 reported by @JDowny
Fix gnutls (again) on Windows by using gnutls_transport_set_errno()
m_ssl_gnutls Call gnutls_transport_set_errno() on Windows only
Fix an extremely low risk crash bug in m_connectban.
Merge pull request #526 from SaberUK/insp20+connectban-fix

[2.0] Fix an extremely low risk crash bug in m_connectban.
Fix thread handle leak in threadengine_win32
Remove unused variables, avoid copies where possible, check empty() instead of size() == 0
Most of these were detected by cppcheck
Workaround for std::list::size() having linear complexity on some implementations
Fix m_randquote with 0 quotes
Initialize local_count
m_geoip Set cc in OnSetConnectClass to the newly created string if it was NULL
m_httpd_acl Reread config on rehash
m_channames Fix iteration in ValidateChans()

Spotted by @Adam-
m_mysql Fix crash on rehash when the database tags have been changed in the config
Update authors
m_nicklock Fix wrong hook in SetPriority()
Remove dead ModeParser code