FDs > 100 no longer closed due to removal of `closefrom(3)`

See comment here: https://github.com/jart/pledge/commit/eb7cc789a3f2ab0fab24f1daf06fd21b61318605#r176532424

(Looks like Github is bugged: It only scrolls to my comment when you open the link in a new tab; if you just click it, it just shows the top of the diff.)

Start with the linked commit and its referenced review comment to understand the removed closefrom(3) behavior. Trace the file-descriptor closing path and reproduce the issue with descriptors above 100. Done means descriptors above 100 are closed again without regressing lower-numbered descriptors.

Written by the indexing model from the issue text.