date: edge cases for modifiers

[ChrisDryden]

Now that we have a library for handling modifiers in #10940, I spent a while fuzzing to see if I could make a list of some follow up tasks for modifier compatibility. These are the only two that I could find:

1. %+ without explicit width adds a sign

examples:

date -d '1999-06-01' '+%+Y'

GNU: "1999"
uutils: "+1999"

2. %_ without explicit width doesn't re-pad to the specifier's default width

examples:

date -d '1999-06-01' '+%_m'

gnu 9.10 gives: " 1"
uutils gives: "1"

This one is a bit more tricky because it would mean that we need to store a mapping for each specifiers default width