Add '#!/hint/bash' headers to non-executable bash files.

This provides a cross-editor hint that the syntax of the file is Bash.

bash_completion.in

+#!/hint/bash
+
 _devtools_compgen() {
   local i r
   COMPREPLY=($(compgen -W '$*' -- "$cur"))

lib/archroot.sh

+#!/hint/bash
+
 CHROOT_VERSION='v4'
 
 ##

lib/common.sh

+#!/hint/bash
+
 # Avoid any encoding problems
 export LANG=C
 

lib/valid-tags.sh

+#!/hint/bash
+
 _arch=(
 	i686
 	x86_64