EDTF intervals with unknown/start end are labeled as open start/end

[kspurgin]

Open end and unknown end mean two different things and should be labeled distinctly.

The following patterns are currently labeled with phrases beginning or ending with "open end" or "open start":

1985-04-12/
1985-04/
1985/
/1985-04-12
/1985-04
/1985

Their display phrases should say "unknown end" or "unknown start" instead.

For example:

../1985 is labeled "open start to 1985" -- CORRECT
/1985 is labeled "open start to 1985" -- WRONG

Lack of .. as the beginning of the range should result in display: "unknown start to 1985"

[kspurgin]

meta-issue: #1748