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
