Add attributes to buffer functions, improve docs

Add GCC attributes to util/buffer.h functions, mostly nonnull.
Clarify the documentation of some of the functions, particularly
around error reporting.
Remove useless $_ to make new perlcritic happy

The latest version of perlcritic added a new test for using an
explicit $_ when use of $_ is implied.  Since I use an explicit
variable where possible, the places this triggers make sense to
simplify.  Do so in tests/perl/module-version-t and in
Test::RRA::Automake.
Remove duplicate license and rra-c-util pointer in Test::RRA

There is a COPYRIGHT AND LICENSE and a SEE ALSO section in the
documentation of all of these modules, so don't duplicate that
information in the header.  It's just one more place that can
get out of date.
Update module versions for the current rra-c-util version