Make "--unshare-user-try" check if /proc is mounted with hidepid

[madaidan]

This is a workaround for https://github.com/containers/bubblewrap/issues/198

It checks if /proc/mounts contains anything related to hidepid.

I'm very new to C so sorry if there's anything wrong. It worked fine when testing.

[rh-atomic-bot]

Can one of the admins verify this patch? I understand the following commands:

• bot, add author to whitelist
• bot, test pull request
• bot, test pull request once

[alexlarsson]

That is a very very rough way of detecting the flags (string compare over entire file). There is some mountinfo parsing in bind-mount.c, i think using that for this would make more sense.

[madaidan]

It is very rough but I don't know how else to detect the flags.