RFR(S): 8245600: Clean up libjli

[Mikael Vidstedt]

Please review this change which cleans up the libjli related files a bit:

JBS: https://bugs.openjdk.java.net/browse/JDK-8245600
webrev: http://cr.openjdk.java.net/~mikael/webrevs/8245600/webrev.00/open/webrev/

Background:

During the review of JDK-8244224 it was noticed that the java_md_solinux* files should be renamed and/or cleaned up. After a quick look I noticed that some relatively straightforward cleanup was possible that removed some obvious duplication.

Details:

* Replace CounterGet and Counter2Micros with a single TimeGetNowMicros. Unify the macosx and unix implementations and put the (only) implementation in java_md_common.c.
* Remove the unused user_dir definition (dead code since https://bugs.openjdk.java.net/browse/JDK-8058407)
* Move the only remaining code - the environ definition and two #includes - from java_md_{solinux,macosx}.h to the unix java_md.h header file
* Rename java_md_solinux.c to java_md.c (which also matches the name of the file on windows)

Testing:

Tier1

Cheers,
Mikael