ftp/sftpgo: SFTP, FTP/S, and WebDAV server [NEW]

Fully featured and highly configurable SFTP server with optional FTP/S
and WebDAV support, written in Go. Several storage backends are
supported: local filesystem, encrypted local filesystem,
S3 (compatible) Object Storage, Google Cloud Storage,
Azure Blob Storage, SFTP.

WWW: https://github.com/drakkan/sftpgo

Approved by: koobs
Differential Revision: https://reviews.freebsd.org/D30661