Windows allows the coexistence of three types of sockets on the same
transport-layer service port, for example, 127.0.0.1:8080, [::1]:8080
and [::ffff:0.0.0.0]:8080
Go don't properly handles this, so we use a ugly hack
Fixes#350
For each user you can now configure:
- TLS certificate auth
- TLS certificate auth and password
- Password auth
For TLS auth, the certificate common name must match the name provided
using the "USER" FTP command
- allow to disable active mode
- allow to disable SITE commands
- add optional support for calculating hash value of files
- add optional support for the non standard COMB command
