contrib/completions: install in $(datadir)/bash-completion/completions

The old /etc/bash_completions.d/ is still available by
installing with something like:
  make install bashcompletionsdir='/etc/bash_completions.d

Thanks to Tomasz Kłoczko for the suggestion (see #413 for details)

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

Author: bgoglin

contrib/completion/Makefile.am

@@ -4,5 +4,5 @@
 
 # This makefile is only reached when building in standalone mode
 
-bashcompdir = $(sysconfdir)/bash_completion.d
-dist_bashcomp_DATA = hwloc-completion.bash
+bashcompletionsdir = $(datadir)/bash-completion/completions
+dist_bashcompletions_DATA = hwloc-completion.bash