chain commands, and let a project refine a catalogue stack

myos build up logs host/fabio runs the three in order and stops at the first
failure, the way make build up logs STACK=host/fabio did. Leading words that
name commands are commands; the first word that is not one starts the stacks.

A stack found in several directories of the stack path is now merged rather
than shadowed, least specific first, so a project drops
stack/postgres/postgres.local.yml next to the catalogue's postgres.yml and
refines it. Settings hooks follow the same order, so a project can redefine a
default the catalogue ships. Neither engine did this before: the project
directory simply hid the catalogue one.

An unknown command now says so and suggests the command to type, instead of
printing the whole usage.

CHANGELOG.md

@@ -20,6 +20,9 @@
 - the project `.env` now wins over `/etc/conf.d/myos`, as documented;
   `MYOS_CONF_PRIORITY=system` restores the previous order
 - `share/make/shim.mk`: make as an optional front end over the same shell code
+- commands chain: `myos build up logs host/fabio`, as make targets did
+- a stack found in several directories of the stack path is merged, project
+  last, so a project refines a catalogue stack instead of replacing it
 - `--color always|never|auto`, and no colour when the output is piped
 - verified under the /bin/sh of Alpine (busybox) and Debian (dash)
 - the make engine still works and is still covered by the golden tests