missing-values: Make mv_add_str() easier to use.

missing-values: Make mv_add_str() easier to use.

Both existing callers of mv_add_str() were applying their own padding to
the string value.  This commit moves the padding into mv_add_str() itself.

This commit also fixes a minor to-do item in the MISSING VALUES command
whereby truncation of too-long missing values was done bytewise rather than
on the basis of full characters.