travis: check spelling of binaries and manpages, use docker

We now build a docker base container based on debian sid (where the very
latest packages are available). That base container is updated once a
month, or whenever travis-build.Dockerfile or debian/control change, but
re-used for subsequent travis runs. While the initial build might take
up to 15 minutes, subsequent builds typically run in a minute or two.

All the different steps that we run on travis are now factored into
separate scripts in the travis/ directory.

Switching to docker should also help with issue #2174.

generate-command-parser.pl

@@ -109,7 +109,7 @@ for my $line (@lines) {
 # Second step: Generate the enum values for all states.
 
 # It is important to keep the order the same, so we store the keys once.
-# We sort descendingly by length to be able to replace occurences of the state
+# We sort descendingly by length to be able to replace occurrences of the state
 # name even when one state’s name is included in another one’s (like FOR_WINDOW
 # is in FOR_WINDOW_COMMAND).
 my @keys = sort { (length($b) <=> length($a)) or ($a cmp $b) } keys %states;