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.

travis/check-safe-wrappers.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+funcs='malloc|calloc|realloc|strdup|strndup|asprintf|write'
+cstring='"([^"\\]|\\.)*"'
+cchar="'[^\\\\]'|'\\\\.[^']*'"
+regex="^([^'\"]|${cstring}|${cchar})*\<(${funcs})\>"
+detected=0
+while IFS= read -r file; do
+    if { cpp -w -fpreprocessed "$file" || exit "$?"; } | grep -E -- "$regex"; then
+        echo "^ $file calls a function that has a safe counterpart."
+        detected=1
+    fi
+done << EOF
+$(find -name '*.c' -not -name safewrappers.c -not -name strndup.c)
+EOF
+if [ "$detected" -ne 0 ]; then
+    echo
+    echo "Calls of functions that have safe counterparts were detected."
+    exit 1
+fi