Convert path to binary to absolute path first

Fix download to file

Show download error message

Add missing space

Log filelist URL to download in --verbose mode.

Fix download

Use std::optional

Use `file_handle` and `std::variant`

Handle static CURL

Bump C++ standard version to C++17 (#12)

s25update.cpp: Fix a readability issue

Fix compatibility with libCURL 7.32+

CI: Run style checks

Fix compatibility with Boost 1.74+

Fix manifest