patch mailsync on macOS; fixes #725

The macOS version of `pidof` doesn't implement the `-s` flag.

bin/mailsync

@@ -14,7 +14,7 @@
 # Run only if user logged in (prevent cron errors)
 pgrep -u "${USER:=$LOGNAME}" >/dev/null || { echo "$USER not logged in; sync will not run."; exit ;}
 # Run only if not already running in other instance
-pidof -s mbsync >/dev/null && { echo "mbsync is already running." ; exit ;}
+pidof $([ $(uname) != Darwin ] && echo '-s') mbsync >/dev/null && { echo "mbsync is already running."; exit ;}
 
 # First, we have to get the right variables for the mbsync file, the pass
 # archive, notmuch and the GPG home.  This is done by searching common profile