net/grive2: Update to 0.5.1-dev

• Use DISTVERSION instead of PORTVERSION for the new version
• Remove CONFLICTS_INSTALL (grive port deleted)
• Fix libgrive/test build after removing doclist api

Noteworthy changes in version 0.5.1-dev

• Added no-remote-new and upload-only modes
• Ignore regexp not persist anymore (note that Grive will still track it to not accidentally delete remote files when changing ignore regexp)
• Added options to limit upload and download speed
• Faster upload of new and changed files. Now Grive uploads files without calculating md5 checksum when file is created locally or when its size changes