RFE: drop libseccomp < 2.5.0 support

[kolyshkin]

Currently, this library has a number of quirks and workarounds to work with older libseccomp. Some of these were removed by 449387b.

Requiring libseccomp >= 2.5.0 should simplify some code.

Related: #99

[rata]

@kolyshkin Don't we care about distros with older versions of libseccomp? For example, in runc we use that (and I know you are aware).

Were you thinking to do it once distros with older versions are unsupported? Or am I missing something?

[pcmoore]

Distros are free to do whatever they like, but understand that from an upstream perspective we no longer support libseccomp < 2.5.x.