ssh-agent: fix file descriptor inheritance (bug #3835)

[ccxcz]

Remove closefrom() call which breaks user-provided command when it relies on inherited file descriptors. This is a regression introduced in commit 66e9868 which introduces closefrom() call closing all file descriptors above stderr (or open socket) which breaks file descriptor inheritance for any processes spawned by ssh-agent, is undocumented behavior, is mismatched from how upstream ssh-agent behaves and has no benefit.

ssh-agent allows specifying arbitrary commands to be run:

1. immediately, conventionally for user session to which the lifetime of ssh-agent is tied to
2. for prompting for passphrases and confirmations using the ssh-askpass protocol

Either of those can rely on pre-opened file descriptors as a means of communication or access control, especially in restricted environments (pledge, capsicum, seccomp-based sandboxes) where arbitrary open() and connect() are not available. When running user-provided commands it's best practice to change the inherited process state as little as possible.