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>

debian/control

@@ -2,7 +2,7 @@ Source: mosh
 Section: net
 Priority: optional
 Maintainer: Keith Winstein <keithw@mit.edu>
-Build-Depends: debhelper (>= 7.0.50), autotools-dev, protobuf-compiler, libprotobuf-dev, dh-autoreconf, pkg-config, libutempter-dev, zlib1g-dev, libncurses5-dev, libssl-dev
+Build-Depends: debhelper (>= 7.0.50), autotools-dev, protobuf-compiler, libprotobuf-dev, dh-autoreconf, pkg-config, libutempter-dev, zlib1g-dev, libncurses5-dev, libssl-dev, bash-completion
 Standards-Version: 3.9.6.1
 Homepage: http://mosh.mit.edu
 Vcs-Git: git://github.com/keithw/mosh.git
@@ -10,6 +10,7 @@ Vcs-Browser: https://github.com/keithw/mosh
 
 Package: mosh
 Architecture: any
+Pre-Depends: dpkg (>= 1.15.7.2)
 Depends: ${shlibs:Depends}, ${misc:Depends}, openssh-client
 Recommends: libio-socket-ip-perl
 Description: Mobile shell that supports roaming and intelligent local echo

debian/mosh.maintscript

@@ -0,0 +1 @@
+rm_conffile /etc/bash_completion.d/mosh 1.2.5~ -- "$@"