Semantic Versioning for sop

This initial series is just the start of semantic versioning for sop itself, inspired by what we've learned from sopv.

Most crucially (from the diff):

sop versioning aims for each subsequent version of the specification to provide two things:

• a reasonable collection of functionality for some consumers of the sop interface, and
• a reasonable next step for a piecemeal implementation of the entire sop interface

This series starts with a simple series, where each level adds the following feature bundles on top of the previous one:

• 1.0: sopv 1.0 + keygen + signing
• 1.1: sopv 1.1
• 1.2: data handling (armor/dearmor/inline-detach)
• 1.3: secret key locking/unlocking

Future feature bundles that don't introduce a semver incompatibility will probably be:

• encryption + decryption
• sopv 1.2 + certification

(though i haven't decided what order to put those in)

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Note that this series also deprecates/removes the apparently unusable sop version --sop-spec option.

Feedback welcome!