RFR: 8349546: Linux support for Kerberos "nativeccache" functionality [v10]

[Nick Hall]

> _Purpose_
> 
> This PR allows Linux based applications using JAAS to acquire Kerberos TGTs natively using the local system's Kerberos libraries/configuration, building on existing support on Windows/MacOSX.
> 
> _Rationale_
> 
> Currently the (pure java) JAAS codebase only supports file-based credential caches (ccaches).  There are many other useful types of ccache accessible via the local system libraries; this change allows credentials to be acquired natively using those libraries, and thus adds support for all other ccache types supported by the local system (e.g. KCM, in-memory and kernel types),  This support already exists on MacOSX and Windows.
> 
> The code change here largely uses the MacOSX code, edited for Linux with associated build system changes. It also adds an appropriate jtreg test which uses some native test helper code to manufacture an in-memory cache, and then uses the new code to acquire these credentials natively.  This has been tested on Linux/Mac and the jtreg test passes on each (I couldn't see any existing tests on MacOSX for this feature).
> 
> Additionally this PR fixes a bug that's existed for a while (see L585-588 in `nativeccache.c`) - without this code, this is a 100% reproducible segfault on Linux (it's unclear why this hasn't affected the Mac JVMs up to now, probably just no calling code that provides an empty list of addresses).  It also fixes a (non problem) typo in the variable name in a function prototype.
> 
> _Implementation Detail_
> 
> Note that there were multiple possible ways of doing this:
> 
> 1) Duplicate the MacOSX `nativeccache.c`, edit lightly for Linux and build a new library on Linux only (`liblinuxkrb5`), leaving MacOSX largely unchanged, but at the expense of this code duplication.
> 
> 2) Create a new shared library used on both platforms with conditional compilation to manage the differences.  This necessitates a library name change on MacOSX and potentially knock-on packaging changes on that platform, which seemed a potentially expensive side-effect.
> 
> 3) Create a shared `nativeccache.c` (using `EXTRA_SRC` in the build) and build separate MacOSX/Linux libraries.  This allows the MacOSX library name to remain unchanged, and only adds a new library in Linux.
> 
> I tried all three options; 3 seemed to be the best compromise all around, although is one of the options that effectively introduces a "no-op" change on MacOSX as a result.  Hopefully the additional jtreg test is sufficient to compensate for this.
> 
> Interested to hear if anyone else...

Nick Hall has updated the pull request incrementally with one additional commit since the last revision:

  Address second set of @erikj79's build comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28075/files
  - new: https://git.openjdk.org/jdk/pull/28075/files/47968fe7..8c8288a3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28075&range=09
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28075&range=08-09

  Stats: 15 lines in 2 files changed: 10 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/28075.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28075/head:pull/28075

PR: https://git.openjdk.org/jdk/pull/28075