GitLab

This updates our port of zlib to the latest at time of writing, going in steps, and testing the result at each stage:

• 1.2.5.3 (having started at 1.2.5 in zlib-1_20)
• 1.2.6.1 (the last of 1.2.6.x series)
• 1.2.7.3 (the last of 1.2.7.x series)
• 1.2.8 (the last of the 1.2.8.x series)
• 1.2.9 (skipped, this was a faulty release obsoleted 2 days later)
• 1.2.10 (skipped, this was a faulty release obsoleted 13 days later)
• 1.2.11 (skipped, this version had a deflate header bug which we need for the RISC OS file attribute additions to work)
• 1.2.12 (also resolves CVE-2018-25032)

And exports 6 of the new functions via modlib (of which 4 are SWIs in the ZLib module).