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.