sysutils/archivemount: update to 0.8.12

Upstream switched to https, so MASTER_SITES changed
to avoid extra request and delay.

Put archivemount more in line with common FreeBSD utilities
providing symlink mount_archive and add pkg-message describing
how it can be used in /etc/fstab.

PR: 237014
Approved by: jirnov@gmail.com (maintainer timeout, 7 weeks)