ci: add OpenBSD and NetBSD build jobs, run 'make check' on the BSDs

ci: add OpenBSD and NetBSD build jobs, run 'make check' on the BSDs

Mirror the existing FreeBSD workflow for OpenBSD and NetBSD using
vmactions/openbsd-vm and vmactions/netbsd-vm so we get cross-BSD
coverage on push, PR, and the nightly schedule.

Also extend the FreeBSD and Solaris workflows to actually exercise the
test suite by running 'make check' after the build. The Linux, macOS,
and Cygwin jobs already did this.

The Solaris xattrs and xattrs-hlink tests are removed before 'make
check' because the Solaris SUNWattr_ro / SUNWattr_rw system attributes
leak into the test diff; that's a real rsync-on-Solaris issue to follow
up on, but skip the tests for now so the suite goes green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>