(sgml-parse-tag-backward): Try and detect

(sgml-parse-tag-backward): Try and detect
when we're starting from within a tag.
(sgml-get-context): When called from inside a tag, do something useful.
Rename the arg now that it's never used for `full' context anymore.
(sgml-calculate-indent): Make `lcon' an argument.
Return nil when we don't know what to do.
If the initial lcon turns out to be wrong, try again.
(sgml-indent-line): If sgml-calculate-indent returns nil, don't indent.