Log Message:
Upgrade mail/courier-unicode to v2.1.

There are currently no pkgsrc packages that depend on this one, so
we don't bother with keeping around the old version.

From the changelog:

2018-07-13  Sam Varshavchik  <mrsam@courier-mta.com>

        * unicode.c: Fix error and validation of valid modified-utf7
        encoding.

2018-07-11  Sam Varshavchik  <mrsam@courier-mta.com>

        * unicode.c: Implement unicode_x_smap_modutf8 pseudo-encoding.

2018-04-27  Sam Varshavchik  <mrsam@courier-mta.com>

        * gcc 8 update, fix assertions. libtool and toolchain updates.

2.0

2017-03-06  Sam Varshavchik  <mrsam@courier-mta.com>

        * Migrate to C++11: remove unicode_char, use char32_t. Replace
        std::vector<unicode_char> with std::u32string.