Support for reading the password from a different file descriptor than standard input

[Earnestly]

It is possible to add a file to a 7z archive from data read on stdin (standard input) using the -si option.

However if the 7z archive is password protected the user has to provide the password on the command-line such as -pPassword which is visible to procfs. This can be demonstrated:

$ 7z -pfoo -sibar a test.7z

# Then, in a new terminal:

$ pgrep -af 7z
2101381 /usr/lib/7zip/7z -pfoo -sibar a hmm.7z
#                          ^~~ visible password.

See also: #33

If 7z could read the password from a different fd (file descriptor) with a theoretical flag like -pfd[N] it would be possible to avoid this problem, e.g.:

data-from-stdin | 7z -pfd9 9< <(printf %s "$passphrase") -sinew/file a test.7z

[Earnestly]

I have implemented this idea as a rough proof of concept. As a bonus, the password is not echoed to the terminal. Perhaps later I can clean it up a bit, add error checking and turn it into a PR. It shouldn't conflict with #33.

Edit: See #186