Improve Debian bash-completion support

* Autoconf queries pkgconf for bash-completion dir-- resolves
  lintian's `package-install-into-obsolete-dir` diagnostic
* Use improved completion snippet from
  <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=782169>
* Remove old /etc/bash_completion.d/mosh "conffile" with
  dh-maintscript + dpkg-maintscript-helper

Closes #628.

Signed-off-by: John Hood <cgull@glup.org>

configure.ac

@@ -369,6 +369,13 @@ AC_CHECK_DECL([IUTF8],
 # Checks for protobuf
 PKG_CHECK_MODULES([protobuf], [protobuf])
 
+# Bash completion needs to ask where it goes if >= 2.0 is installed.
+AS_IF([test "$install_completion" != no],
+  [PKG_CHECK_MODULES([BASH_COMPLETION], [bash-completion >= 2.0],
+     [completions="`pkg-config --variable=completionsdir bash-completion`"],
+     [completions="${sysconfdir}/bash-completion.d"])
+   AC_SUBST([completions])])
+
 AC_CONFIG_FILES([
   Makefile
   src/Makefile