Improved Linux and MacOS support

This release has much better support for Linux, and for the first time supports MacOS.
The build and installation procedures are the same for MacOS as for Linux, collectively referred to as Unix.

Other significant improvements

• All C tools now build in Unix without warnings.
• Unix make install is now more standard, using the Unix install command internally.
  It can also be dry-runned by running make -n install
• install: New script for installing select tools in Unix. (Contrary to make install which installs everything.)
• update.exe: Several new options, and a few old ones renamed for consistency:
  Renamed switches -e|--erase as -c|--clean. (By analogy with make clean.)
  Renamed option -T|-resettime as -R|-resettime.
  Added switches -T|--tree, independent of -E|--noempty. (As the 2018 change linking the two features did more harm than good.)
  Renamed options -S|--showdest as -D|--dest, and added -S|--source to explicit the default behavior.
  Added options -C|--command to display the equivalent shell commands. (And thus display both the source and dest. files.)
  Added option -B|--nobak to skip backup and temporary files. (*.bak, ~, ##)
• dirc.exe: Added option -B to skip backup and temporary files.
• halve.bat: New filtering function for use with 12.bat.
• sml2.exe: Now preserves head spaces, if any. This allows successfully converting indented blocks of XML.

Major bug fixes

• update.exe: Fixed a serious usability issue when the target was a link to a directory.
• backnum, dirc, dirsize, rd, redo, update, zap: Fixed an issue which sometimes caused failures in Unix.
• which: Fixed wildcards search in Unix.

Please as usual many small improvements and minor bug fixes