Allow rc.topomap/linktest to use gzip'd versions of ltmap/ltpmap files.
If $ETCDIR/ltmap-gzip exists on a clientside node, rc.topomap will only download the gzip'd versions of ltmap/ltpmap into $BOOTDIR, and linktest.pl will use them. Those files are TMLTMAPGZ() and TMLTPMAPGZ(). This is important for multi-thousand node exps, where the lt*map files easily grow to 250MB or more (and are compressible to 25:1 or so!); saves CoW virtual disk blocks and raw disk space. And now that commit 67cd8518 means nodetype no longer uses ltpmap, linktest is the only consumer of lt*map files.
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