[Fix #419] start gpg-agent also with ssh-support off

and add a note on the variables exported by this module

modules/gpg-agent/README.md

@@ -12,6 +12,14 @@ To enable SSH-Agent protocol emulation, add the following line to *zpreztorc*:
 
     zstyle ':prezto:module:gpg-agent' ssh-support 'yes'
 
+Variables
+---------
+
+Please note that if you use this agent, the variables will only be visible to
+other instances of zsh. Should you use a non-shell mail user agent such as
+thunderbird, make sure it inherits the variables. For an example how to
+achieve this, have a look at issue #419 in the [issue tracker][2].
+
 Authors
 -------
 
@@ -22,4 +30,3 @@ Authors
 
 [1]: http://linux.die.net/man/1/gpg-agent
 [2]: https://github.com/sorin-ionescu/prezto/issues
-

modules/gpg-agent/init.zsh

@@ -33,7 +33,7 @@ function _gpg-agent-start {
 # 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' || {
+  ps --no-headers -C 'gpg-agent' || {
     _gpg-agent-start
   }
 else