net/netatalk[34]: Add a new port, netatalk4

Netatalk4 is the latest release of the Netatalk Open Source macOS AFPD
file server suite.

For now, both netatalk3 and netatalk4 will remain, but it is becoming
clear, netatalk4 is The Way™, and netatalk3 should be considered
deprecated.

Netatalk4 has had several releases already, so the underlying afpd
server should be quite stable. Note that since FreeBSD removed
AppleTalk (ddp) support a while ago, that part of netatalk4 is not
enabled.

See https://github.com/Netatalk/netatalk/releases for a summary of all
changes since netatalk3.