Update blacklist support in ftpd to clarify fd usage

The ftp daemon dups the control socket to stdin and uses that fd
throughout the code. Clarify this usage slightly by changing from
explicit use of "0" for the fd to a variable, to make it clear what
the zero represents in the non-blacklist code. Make the
blacklist_notify routine use STDIN_FILENO so as to have less of a
"magic number" feel to the code.

Reviewed by: cem
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6716