RFR: 8348998: Split out PreInit.gmk from Init.gmk

[Magnus Ihse Bursie]

The Init.gmk (and, correspondingly, InitSupport.gmk) is really two files in one: First it is called without $(SPEC), and does the work needed to locate a SPEC (and sanity check arguments), and then it calls itself again with a SPEC value.

This is two completely different tasks, and there is no reason it should be put in the same file. It just makes the file longer and harder to read and understand. 

This work is done as a preparation for fixing https://bugs.openjdk.org/browse/JDK-8292944. The code was just to hard to follow to be able to fix it without going insane.

Also, GitHub is spectacularly bad at showing the diff for this patch! I think it might be easier to review if you do it commit by commit, or possibly using the webrev frame view.

-------------

Commit messages:
 - Update/clarify comments, and make some small style adjustments.
 - Rename BothInitSupport to LogUtils
 - Invert if block so the common case comes first. Also some additional cleanup.
 - Rename SpecInit to just Init
 - Remove Init.gmk
 - Remove InitSupport.gmk
 - Split InitSupport.gmk into three parts
 - Split Init.gmk into two

Changes: https://git.openjdk.org/jdk/pull/23355/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23355&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348998
  Stats: 1488 lines in 7 files changed: 692 ins; 621 del; 175 mod
  Patch: https://git.openjdk.org/jdk/pull/23355.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23355/head:pull/23355

PR: https://git.openjdk.org/jdk/pull/23355