47358: Add a ZSH_XTRACEFD environment variable to redirect debug traces

[pawamoy]

Just opening for code review purposes. Patches must be sent by email to the zsh-workers mailing list. Ref of previous discussions around ZSH_XTRACEFD: 47358 (for example see https://www.zsh.org/mla/workers//2020/msg01177.html).

[pawamoy]

Initially opened there by mistake: zsh-users#113 (cross-referencing in case there's activity over there as well).

[pawamoy]

Interesting comments:

https://www.zsh.org/mla/workers/2020/msg00611.html

The problem is if we fopen() the file descriptor to use stdio as output, we can either
leak the entire FILE, not opened by the user, or we can close the entire FILE.

In that case we should be doing the fopen() on a dup() of the
descriptor, and fclose()ing the FILE.

If it is important that fileno(xtrerr) == $ZSH_XTRACEFD, then we should

1. dup() the descriptor to save a copy
2. fopen() the original
3. after fclose(), dup2() the copy back to the original
4. close() the copy

However, I'm not sure it's necessary to be that convoluted.