Support GNU gettext message catalogs (.mo) in dcgettext(), dcngettext(), and bindtextdomain()

[bertysentry]

Current state

Since #80 / PR #527, dcgettext(), dcngettext(), and bindtextdomain() exist but perform no actual translation: text is returned untranslated and dcngettext() applies the English plural rule. This matches gawk built without gettext — gawk's own test suite accepts this: its nlstringtest target compares against nlstringtest-nogettext.ok first, and Jawk's GawkLocaleIT.test_nlstringtest passes against that baseline.

Proposed feature

Implement real message catalog support so the i18n functions translate like gawk built with gettext:

• Parse the GNU .mo binary catalog format (simple: magic 0x950412de, offset tables of original/translated strings, optional hash table).
• Resolve the catalog path as gettext does: <binding directory>/<locale>/LC_MESSAGES/<domain>.mo, with the binding from bindtextdomain(), the domain from the argument or TEXTDOMAIN, and the locale from the CLI --locale option / JVM default locale (gawk uses GAWKLOCALE/LC_ALL).
• Plural selection in dcngettext() per the catalog's Plural-Forms: header, which requires evaluating its small C-like expression (e.g. nplurals=2; plural=(n > 1); for French).
• Note the interaction with Reincorporate Printf4J into Jawk to fully implement AWK's printf #528: translated format strings commonly use positional conversions (%2$s remplace %1$s. in gawk's French catalog), so full fidelity also needs positional printf argument support.
• Sandbox: catalog loading reads files at runtime, so it must respect SandboxedAwk restrictions (probably disabled under sandbox).

Acceptance test

gawk's own fixture: running nlstringtest.awk with the compiled catalog test/fr/LC_MESSAGES/nlstringtest.mo (from the gawk source tree) under a French locale must produce the translated nlstringtest.ok:

2 morceaux de gateau
%2$s remplace %1$s.
%2$s remplace %1$s.

EUR remplace FF.

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