feat: loading config file from dir of the executable file path

[dake0805]

when looking for config file, adding another path besides

• ./
• /etc/webdav/

which is the path to the executable file webdav or webdav.exe,

So I can put the binary file and the config file together, and running webdav with this config file from any other working directory.

like the example in pic, config.yml set the port to 80 and the program read it correctly.

test passed on Windows 11 and Ubuntu 20.04 (WSL2)