[ssh] Update init.zsh

based on https://stackoverflow.com/a/48509425/2654518, which is based on
http://rabexc.org/posts/pitfalls-of-ssh-agents

Current code has a few issues: depends on `SSH_AUTH_SOCK` and
`SSH_AGENT_PID` env variables, which might not be available in every
shell session; and tries to create a new socket for agent-forwarding by
checking `SSH_AUTH_SOCKET` instead of `SSH_AUTH_SOCK`.

Also, it's safer to create the env file with 066 mode and in the user
home directory.

About not using `$` inside `(( ))`, this is what the the section
ARITHMETIC EVALUATION in zshmisc(1) says:

> Named parameters and subscripted arrays can be referenced by name
> within an arithmetic expression without using the parameter expansion
> syntax.

And according to http://www.bash2zsh.com/zsh_refcard/refcard.pdf:

> `var` (does not require `$` in front unless some substitution e.g.
> `${#var}` is needed, `$` is error if `var` is to be modified)

Closes #292

modules/ssh/init.zsh

@@ -7,40 +7,28 @@ if (( ! ${+commands[ssh-agent]} )); then
   return 1
 fi
 
-# use a sane temp dir; creating 1k ssh-* files in /tmp is crazy
-if [[ ${TMPDIR} ]]; then
-  local ssh_env=${TMPDIR}/ssh-agent.env
-  local ssh_sock=${TMPDIR}/ssh-agent.sock
-else
-  # create a sane tmp dir at /tmp/username
-  mkdir -p /tmp/${USER}
-  local ssh_env=/tmp/${USER}/ssh-agent.env
-  local ssh_sock=/tmp/${USER}/ssh-agent.sock
-fi
+ssh-add -l &>/dev/null
+if (( ? == 2 )); then
+  # Unable to contact the authentication agent
 
-# start ssh-agent if not already running
-if [[ ! -S ${SSH_AUTH_SOCK} ]]; then
-  # read environment if possible
-  source ${ssh_env} 2> /dev/null
+  # Load stored agent connection info
+  local ssh_env="${HOME}/.ssh-agent"
+  [[ -r ${ssh_env} ]] && source ${ssh_env} >/dev/null
 
-  if ! ps -U ${LOGNAME} -o pid,ucomm | grep -q -- "${SSH_AGENT_PID:--1} ssh-agent"; then
-    eval "$(ssh-agent | sed '/^echo /d' | tee ${ssh_env})"
+  ssh-add -l &>/dev/null
+  if (( ? == 2 )); then
+      # Start agent and store agent connection info
+      (umask 066; ssh-agent >! ${ssh_env})
+      source ${ssh_env} >/dev/null
   fi
 fi
 
-# create socket
-if [[ -S ${SSH_AUTH_SOCKET} && ${SSH_AUTH_SOCKET} != ${ssh_sock} ]]; then
-  ln -sf ${SSH_AUTH_SOCKET} ${ssh_sock}
-  export SSH_AUTH_SOCK=${ssh_sock}
-fi
-
-# load ids
-if ssh-add -l 2>&1 | grep -q 'no identities'; then
+# Load identities
+ssh-add -l &>/dev/null
+if (( ? == 1 )); then
   if (( ${#zssh_ids} > 0 )); then
     ssh-add "${HOME}/.ssh/${^zssh_ids[@]}" 2> /dev/null
   else
     ssh-add 2> /dev/null
   fi
 fi
-
-unset ssh_{sock,env}