strv: add strv_split_colon_pairs function

strv: add strv_split_colon_pairs function

Given a string in the format 'one:two three four:five', returns a string
vector with each word. If the second element of the tuple is not
present, an empty string is returned in its place, so that the vector
can be processed in pairs.

[zjs: use EXTRACT_UNESCAPE_SEPARATORS instead of EXTRACT_CUNESCAPE_RELAX.
This way we do escaping exactly once and in normal strict mode.]