core.whitespace: split trailing-space into blank-at-{eol,eof}

core.whitespace: split trailing-space into blank-at-{eol,eof}

People who configured trailing-space depended on it to catch both extra
white space at the end of line, and extra blank lines at the end of file.
Earlier attempt to introduce only blank-at-eof gave them an escape hatch
to keep the old behaviour, but it is a regression until they explicitly
specify the new error class.

This introduces a blank-at-eol that only catches extra white space at the
end of line, and makes the traditional trailing-space a convenient synonym
to catch both blank-at-eol and blank-at-eof.  This way, people who used
trailing-space continue to catch both classes of errors.

Signed-off-by: Junio C Hamano <gitster@pobox.com>