Copy and patch readpassphrase.c rather than maintaining a fork #1433

#1431 (and #1432, and #1437, sigh) synced a heavily outdated readpassphrase.c. It will again be forgotten soon.

As I mentioned in #1431 (somewhat edited):

The diff to upstream currently is a complete mess and could surely be quite a bit cleaner. The various defines added to the top probably should go into readpassphrase.h and I have no idea what the reshuffling of the includes is all about. That would then leave the return value handling via bytes_written which is a bit annoying (could perhaps be hacked around with a wrapper/#define for write in this file), and then some minor things.

There's __pledge_open() which could probably just be a compat shim.

This will need a bit of fiddling, but it should not be too hard.