lavf/libssh: support reading config from ~/.ssh/config

libssh provides a function for parsing ~/.ssh/config for ssh connection parameters like user, hostname, identity file
and port. This patch makes ffmpeg use this function to take parameters from the config file for everything that's not
explicitely set in the url. It also supports host aliases, i.e. using a shorthand in the url and replacing it with the
hostname / IP address specified for the shorthand in the config file.

Signed-off-by: Florian Jacob <projects+ffmpeg@florianjacob.de>
Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>