minirst: improve layout of field lists
Before, we used the padding following the key to compute where to wrap
the text. Long keys would thus give a big indentation. It also
required careful alignment of the source text, making it cumbersome to
items to the list.
We now compute the maximum key length and use that for all items in
the list. We also put a cap on the indentation: keys longer than 10
characters are put on their own line. This is similar to how rst2html
handles large keys: it uses 14 as the cutoff point, but I felt that 10
was better for monospaced text in the console.