mbt 2.1.0

Load module attributes are declared, not inherited (cc370#100).

• rent, reus, refr on [[module]]/[[test]]. A declared attribute is passed to ld370 in both directions (--rent/--norent …), so the module no longer depends on ld370's default — RENT+REUS until now, neither (IEWL's) in a later cc370 release. norent/noreus still work, with a warning.
• No writable data in a rent = true module: before linking, make scans the C sources (mbtmoddata.py, #91). cc370 keeps writable statics in the module, and a RENT module LINKed by several tasks is one copy — measured on MVS with three server tasks sharing one copy of a module. Writable data in a rent = true module stops the build; a module that is RENT only by ld370's default, or ac = 1, gets a warning.
• Warnings from the configuration step now reach the build log.

What a project should do: declare rent/reus for every module — rent = true only where the check finds no writable data.

Since v2.0.0 also: -Wall -Wextra -Werror by default (#133); relink when a linked library changes (#103); the .d escape after a failed compile (#134); read timeouts as mbt errors (#108); prerelease moves only the owner's tags (#85); libc370 in cc370's second sysroot (#144); the v2 output baseline and behaviour inventory for mbt 3 (docs/v2-baseline.md, docs/v2-behaviours.md).