kernel-doc: html mode struct highlights

Johannes Berg reported that struct names are not highlighted
(bold, italic, etc.) in html kernel-doc output.  (Also not in
text-mode output, but I don't see that changing.)

This patch adds the following:
- highlight struct names in html output mode
- highlight environment var. names in html output mode
- indent struct fields in the original struct layout

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>