cmd/snap-confine: use defensive argument parser

[zyga]

This switches snap-confine to the far more tested and resilient argument
parser. This fixes a possible attack vector where a malicious
application would fork/exec snap-confine with especially crafted
argument vector.

Signed-off-by: Zygmunt Krynicki zygmunt.krynicki@canonical.com

[stolowski]

Looks very nice, readability is much better than the original. One minor nitpick (a general comment rather) - IMHO it would be good to make even simple if-statements such as:

if (cond)
do_something();

into:

if (cond) {
do_something();
}

It just feels good especially for security-sensitive code (but surely in this particular change it's always followed by die(), so no real risks).

[niemeyer]

This should be something like this instead:

err = sc_parse_args(&argc, &argv, &args);
sc_die_on_error(err);

The usage error may be an actual error inside the respective logic instead of just a code:

error *usage_error = errorf("Usage: snap-confine <security tag> <executable>")

[zyga]

Done.

[chipaca]

LGTM. Not sure github will let you land it while gustavo's changes-requested is still there though.