Improve man -k support.

Improve man -k support.
(Man-reference-regexp): Accpet spaces between
`Man-name-regexp' and `Man-section-regexp'.
(Man-apropos-regexp): New variable.
(Man-abstract-xref-man-page): Use value for `Man-target-string'
if available.
(Man-highlight-references, Man-highlight-references0): Handle
the case when `Man-arguments' includes "-k".
(Man-highlight-references0): Rename the argument `TARGET-POS' to
`TARGET'. `TARGET' can be a number, function or nil.