value-parser: Make parse_value() accept variable's print format also.

value-parser: Make parse_value() accept variable's print format also.

Previously commands such as VALUE LABELS required numeric values to
be given as plain numbers, but this makes it difficult to add
meaningful value labels for variables with date and time formats.
This commit allows values for this command and a few others to be
given in a variable's print format instead.

Bug #18497.