Posix pipe_out is missing uses_handles and get_used_handles

I'm using boost 1.80. I haven't checked if a later development version includes a fix already.

I noticed that file handles were leaking to exec children. This caused some parallel children's hand because leaked pipes stayed open after the source process had exited. When I was trying to figure out how to close the extra file descriptor I noticed limit_handles. limit_handles promises to prevent file descriptors from leaking. Too bad it seemed to break all pipes for children. I managed to figure out that the issue was hidden in the posix pipe_out implementation. All pipe_out file descriptors are being closed.

I tested locally that inheriting from uses_handles and implementing get_used_handles makes my code work.

Inspect the POSIX pipe_out implementation and compare it with the uses_handles and get_used_handles behavior described in the issue. Reproduce the interaction with limit_handles and child pipes, then verify that pipe descriptors remain available to children without leaking unrelated file descriptors.

Written by the indexing model from the issue text.