Fix issues removing autojoin channels

- Makes remove case insensitive
- Fixes crashing when channel not found
Improve displaying errors in servlist
Fix warnings like usual
python: Scan for non-NULLs in print events

Closes #916
Closes #915
sysinfo: Use less ambiguous IEC prefixes in storage measurements

Closes #922
Reset all nick entry errors in servlist when fixed
Use utf-8 variant of strftime to format log file paths.

strftime assumes the format string is in locale encoding, which mangles log file paths that are in utf-8.

Fixes #767
Fixes #945
Don't convert the result of g_get_user_name / g_get_real_name from locale to utf-8 on Windows. They return utf-8 already.
Use the right API to get the path to Roaming AppData and Downloads directories, and actually convert it to utf-8 instead of using it directly.
Merge branch 'master' of https://github.com/hexchat/hexchat.git