bash-completion: improve compatibility with zsh

When running bash-completion's completion functions in zsh, the
`_init_completion` and `_get_first_arg` functions are not defined in
`bashcompinit`. Defining dummy functions allow using auto-completion
in zsh although its not as accurate.

Without _get_first_arg, the completion doesn't detail the options
specific to that command and lists all of them.

Author: deribaucourt

bash-completion

@@ -15,6 +15,13 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+if [ -z "$BASH_VERSION" ]; then
+	_init_completion() {
+	}
+	_get_first_arg() {
+	}
+fi
+
 _cqfd() {
 	local cur prev words cword
 	_init_completion || return