mkrockspecs: compatibility with lyaml > 3.

lyaml > 3 returns all documents in a YAML stream as a table, not
just the first document.
* src/mkrockspecs.in: Detect lyaml 4+ by whether `require "yaml"`
works, or else wrap the lyaml.load function return value in a
table.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>