Fix comment at end of util/buffer.h

Cut and paste error from when this file came from INN.
Fix _RRA_MACRO_IN6_ARE_ADDR_EQUAL_SOURCE comment

Was referencing the wrong parent macro.
Clarify leading comments in util/buffer.*

Use the same terminology in both files, and make it a bit clearer.
Annotate buffer_append_vsprintf tests

Mention in a comment before the test block for that function.
Check return status of ioctl for fdflag_nonblocking

In the ioctl fallback for fdflag_nonblocking, check the return
status of ioctl properly.  (This fallback is probably never used
in practice.)  Thanks to Julien ÉLIE for the report.
Add comment for why fdflag_close_exec doesn't use socket_type
Add test and utility for module versions in Perl dists

Add a style/module-version.t test and helper script for regular Perl
modules.  This is similar to tests/perl/module-version-t except that
it expects a Perl module layout instead of Perl modules embedded in a
larger distribution, does not do the Automake integration, and
supports the Perl 5.12 package syntax.  (Common code should someday be
factored out into Test::RRA.)