Introduce compatibility and convenience targets, local.mk template, add "helpall"

Introduce compatibility and convenience targets, local.mk template, add "helpall"

* Makefile: new target "helpall" to document all targets, while "help"
  continues to show a brief subset.

* default.mk: add cutlines for sed to produce local.mk with.

* targets.mk: add target "local.mk" to produce an (empty) local.mk
  configuration template when it isn't already present.  Ignore any
  error when making this target since sed might not be present on all
  systems.  Redefine target "update" to not include testing, similarly
  add target "update2" to additionally install without test.  Add
  targets "up0" to stop after git pull and "up1" to stop after test,
  while "up2" continues to do everything and then installs.  Complete
  .PHONY target list.  Add "refcard" target for compatibility with old
  make.  Clean contrib in "cleanall", too.