dbus-launch: Replace slashes in DISPLAY if present

dbus-daemon fails to autolaunch with X11 on macOS 10.8+ because XQuartz
(the X11 package for macOS) provides a value for `$DISPLAY` that is
not expected by dbus, in that it contains `/` characters. This is
addressed by replacing the invalid path character `/` with `_`.

Resolves: #8
Resolves: #311
11 jobs for macos-x11-bugfix in 9 minutes and 15 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #5452833
build:cmake

00:03:52

passed #5452830
build:debug

00:09:14

manual #5452835
allowed to fail manual
build:i686-w64-mingw32-cmake
passed #5452834
build:i686-w64-mingw32-debug

00:03:29

manual #5452832
allowed to fail manual
build:legacy
passed #5452829
build:production

00:05:43

manual #5452831
allowed to fail manual
build:reduced
manual #5452838
allowed to fail manual
build:stretch
passed #5452836
build:x86_64-w64-mingw32

00:03:07

passed #5452837
build:x86_64-w64-mingw32-cmake-debug

00:07:57

manual #5452839
allowed to fail manual
build:xenial