Enable SSL on standard and well-known HTTP headers

Enable SSL on standard and well-known HTTP headers

Look for the `Forwarded` header with `proto=https`, as specified
in RFC7239 (strictly expecting no whitespace around the = sign).
This also add similar support for `X-Forwarded-Proto: https`,
`Front-End-Https: on` and `X-Forwarded-Ssl: on` (all case-sensitive).

Also add some documentation about this, and an NginX configuration
example, in INSTALL.txt

This should fix #757.

Signed-off-by: Olivier Mehani <shtrom+friendica@ssji.net>