enable seccomp by default

After some month of using zathura in strict sandbox mode without discovering new bugs, I believe seccomp can be enabled by default in the next release.

Since the default sandbox value is set to "normal" which should not interfere with normal operation anyway, new bugs resulting from the use of seccomp will likely only affect thuse who have the sandbox value set to "strict".

Before new releases are published I would advise for a quick test in strict sandbox mode to discover new bugs resulting from new functionality or changed library behavior. I will try to support this as best I can and periodically test the development branch to make sure no new bugs are introduced through seccomp.