devel/tevent016: Talloc based event loop library

tevent is a simple library that can handle the main event loop for an
application. It supports three kinds of events: timed events, file
descriptors becoming readable or writable and signals.

Talloc is used for memory management, both internally and for private
data provided by users of the library.

This port is needed for Samba 4.20

This commit is just a repocopy. I will update the port to the correct
version and connect it to the build in the next commit.

PR: 280533
Sponsored by: Klara, Inc.
Approved by: 0mp (mentor)
Approved by: samba (0mp, kiwi)
Differential Revision: https://reviews.freebsd.org/D48658