(apropos-match-face): Use `secondary-selection' rather

(apropos-match-face): Use `secondary-selection' rather
than `highlight' to distinguish it from mouse-face highlighting of
hyperlinks.
(apropos-mode-map): Rename from `apropos-local-map'.
(apropos-mode): Set it rather than have a local-map that made RET
locally unusable when copied to other buffer.
(apropos-print): Use it.  When there is only one property, show
what it is.  Remove superfluous `save-excursion', thus making help
commands' return-message be correct.
(apropos-print, apropos-describe-plist): `print-help-return-message'
like help commands.