Remove "standard" cross-version from aether and assemble.

The "standard" cross-versioning level is defined in a precise
manner only for the "sbt" build system, in which it means:
"pick what sbt would normally select depending on the user's
choice of scalaBinaryVersion".

For the other build systems we may work out an approximation
of the same behavior, but it is in general more clear to
directly select one of "disabled", "binary", "full" instead.
Correctly support "binary" cross-version in the Scala build system

The property "maven.version.number" is used to describe the full
scala version number of the resulting artifacts; however, a separate
property scala.binary.version is used to create the cross-version
suffixes in the pom files.

As long as the compiler was generated inside an "Assemble" section,
that would not make a difference, since the suffix was stripped
away and replaced while assembilng, but in a bootstrap process that
requires a binary cross-version, that is important.
Allow "standard" as a cross-version level for Scala

If "standard" is selected, the binary version will be picked
from the version.properties file; that may however be misaligned
with the version selected in the dbuild configuration file
(therefore issue a warning).
Additional docs concerning the cross-version setting.
Also print version and commit on console, if debugging enabled
Vastly better debugging printout of the vars section
Add environment variables debugging output
Merge pull request #153 from cunei/wip-mix-fix-6

Mix fix 6