[patch] OpenBSD's usermod has no -a

[Matthew Martin <phy1729@xxxxxxxxx>]

OpenBSD's usermod has no a flag. The below patch removes it from
completion.

diff --git a/Completion/Unix/Command/_user_admin b/Completion/Unix/Command/_user_admin
index 31ccbf1..3653308 100644
--- a/Completion/Unix/Command/_user_admin
+++ b/Completion/Unix/Command/_user_admin
@@ -44,8 +44,8 @@ if [[ $service = user* ]]; then
 	"(-U -p)-L[lock user's password]"
 	"(-L -p)-U[unlock user's password]"
     )
+    [[ $OSTYPE = openbsd* ]] || args+=('-a[append groups]')
     args+=(
-      '-a[append groups]'
       '-l[specify new user name]:new username'
       ':username:_users'
     )