A common set of functions instead of many separate implementations for
creating buffers that can grow when appending data to them. Existing
functionality has been ported over.

In my early basic testing, the total number of allocations seem at
roughly the same amount as before, possibly a few less.

See docs/DYNBUF.md for a description of the API.

Closes #5300

Closes #5306

Improved estimation of expected_len and updated related comments;
increased strictness of QNAME-encoding, adding error detection for empty
labels and names longer than the overall limit; avoided treating DNAME
as unexpected;

updated unit test 1655 with more thorough set of proofs and tests

Closes #4598

It was used (intended) to pass in the size of the 'socks' array that is
also passed to these functions, but was rarely actually checked/used and
the array is defined to a fixed size of MAX_SOCKSPEREASYHANDLE entries
that should be used instead.

Closes #4169

Most headerfiles end with a /* <headerguard> */ comment, but it was
missing from some. The comment isn't the most important part of our
code documentation but consistency has an intrinsic value in itself.
This adds header guard comments to the files that were lacking it.

Closes #3158
Reviewed-by: Jay Satiro <raysatiro@yahoo.com>
Reviewed-by: Daniel Stenberg <daniel@haxx.se>

Ideally this will fix the reversed order shown in SPARC tests:

  resp 8: Expected 127.0.0.1 got 1.0.0.127

Closes #3091

Closes #2668