security/doas: Update 6.3p13 => 6.4

Changelog:

• Fix some typos in the documentation.
• Cause doasedit to better handle shell variables and find the best text editor for altering files.

Improve port:

• Move BINMODE=4755 to plist: "@(,,4755) bin/doas" - BINMODE is applied universally (to all programs and scripts). This means helper scripts like doasedit, which are meant to be run as a regular user, were also installed as setuid. This is a potential security issue.
• Move plist to file.
• Add "@sample" for "etc/doas.conf.sample".

PR: 293628
Sponsored by: UNIS Labs
MFH: 2026Q2
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>