adns_str* etc.: Return NULL rather than crashing, and docs

adns_str* etc.: Return NULL rather than crashing, and docs

Previously, calling these functions with arbitrary values would cause
them to do a struct member lookup on a null pointer.  Now they return
the pointer instead.

Also, some users have been confused by the docs, and have passsed, for
example, adns_s_max_misconfig.  This is also not permitted.  Clarify
that only values actually returned by adns are allowed.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>