Hooks calling snapctl are broken with just Seccomp enabled

[morphis]

Related bug reports:

• https://bugs.launchpad.net/snappy/+bug/1674193
• https://bugs.launchpad.net/snapd/+bug/1674484
• https://bugs.launchpad.net/snappy/+bug/1644573

The last bug (LP #1644573) is the most relevant for this.