Use $GNUPGHOME in GPG agent startup

Use the value of $GNUPGHOME, to facilitate moving GPG's home.

This lets users who keep their GPG keyring on external devices or alternate
folders use the gpg-agent autostarting, even if $HOME/.gnupg does not exist.

modules/gpg-agent/init.zsh

@@ -0,0 +1,46 @@
+#
+# Provides for an easier use of gpg-agent.
+#
+# Authors:
+#   Florian Walch <[email protected]>
+#   Sorin Ionescu <[email protected]>
+#   neersighted <[email protected]>
+#
+
+# Return if requirements are not found.
+if (( ! $+commands[gpg-agent] )); then
+  return 1
+fi
+
+# Make sure to use the $GNUPGHOME first.
+_gpg_env="${GNUPGHOME:-$HOME/.gnupg}/gpg-agent.env"
+
+function _gpg-agent-start {
+  local ssh_support
+
+  zstyle -b ':prezto:module:gpg-agent' ssh-support 'ssh_support' \
+    || ssh_support=''
+
+  gpg-agent \
+    --daemon \
+    ${ssh_support:+'--enable-ssh-support'} \
+    --write-env-file "${_gpg_env}" > /dev/null
+
+  chmod 600 "${_gpg_env}"
+  source "${_gpg_env}" > /dev/null
+}
+
+# Source GPG agent settings, if applicable.
+if [[ -s "${_gpg_env}" ]]; then
+  source "${_gpg_env}" > /dev/null
+  ps -ef | grep "${SSH_AGENT_PID}" | grep -q 'gpg-agent' || {
+    _gpg-agent-start
+  }
+else
+  _gpg-agent-start
+fi
+
+export GPG_AGENT_INFO
+export SSH_AUTH_SOCK
+export SSH_AGENT_PID
+export GPG_TTY="$(tty)"