net/libbgpdump: Update 1.6.0 => 1.6.2, improve port

Changelog:
https://github.com/RIPE-NCC/bgpdump/blob/v1.6.2/ChangeLog

• Replace PORTVERSION with DISTVERSION.
• Switch upstream to GitHub.
• Add LICENSE_FILE.
• Fix warnings from portclippy.
• Add soname.
• Use INSTALL_LIB instead of INSTALL_DATA+STRIP_CMD for libs.
• Optimize includes installation.
• Install include/bgpdump_mstream.h.
• Move docs and examples installation to do-install-DOCS-on and do-install-EXAMPLES-on.
• Parametrize libbgpdump with PORTNAME.

PR: 272484
Approved by: mjl@luckie.org.nz (maintainer)