If the user defines identities in `:prezto:module:ssh:load`, always try
to load them, even on macOS. On macOS, also try to load Keychain
managed identities. (Assume if a user adds an identity to Keychain then
the use would want those identities automatically loaded.)
* Remove persistent SSH auth socket.
* Make ps|grep more robust and POSIX compliant.
* On macOS, use `-A` switch to "add identities to the agent using any
passphrase stored in the user's keychain."
create ssh_agent_env with current user id in file name to avoid collisions with other users
create ssh_agent_sock with current user id in file name to avoid collisions with other users
If `SSH_AGENT_PID` is unset, grep will succeed if another ssh-agent is
running as it will match the string `ssh-agent` where the PID was an
empty string. Set a default value for grep to a value that will never
match if unset, i.e. -1.
Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
