ssh: check ssh-askpass presence under Linux

Check the presence of `ssh-askpass`, if `/usr/lib/ssh/x11-ssh-askpass`
is present I use it otherwise I require the passphrase in the standard
method

modules/ssh/init.zsh

@@ -39,10 +39,11 @@ fi
 # Load identities.
 if ssh-add -l 2>&1 | grep -q 'The agent has no identities'; then
   zstyle -a ':prezto:module:ssh:load' identities '_ssh_identities'
-  if (( ${#_ssh_identities} > 0 )); then
-    ssh-add "$_ssh_dir/${^_ssh_identities[@]}"  < /dev/null 2> /dev/null
+  # Check for Linux system and ssh-askpass presence
+  if [[ "$OSTYPE" == linux* ]] && [[ ! -a /usr/lib/ssh/x11-ssh-askpass ]]; then
+    ssh-add "${_ssh_identities:+$_ssh_dir/${^_ssh_identities[@]}}"  2> /dev/null
   else
-    ssh-add  < /dev/null 2> /dev/null
+    ssh-add "${_ssh_identities:+$_ssh_dir/${^_ssh_identities[@]}}"  < /dev/null 2> /dev/null
   fi
 fi