Add idempotent brew_launchctl_restart function

Fixes thoughtbot#260

This is a soft error that occurs when Laptop is run and PostgreSQL isn't
currently loaded by `launchclt`, e.g. on the first run of Laptop:

    Starting Postgres ...
    /Users/wellmade/Library/LaunchAgents/homebrew.mxcl.postgresql.plist ->
    /usr/local/opt/postgresql/homebrew.mxcl.postgresql.plist
    launchctl: Error unloading: homebrew.mxcl.postgresql

The fix would probably be to check `launchctl list` to see if the service is
already loaded before calling `launchctl unload`.

This small added complexity probably justifies a function for running
appropriate `launchctl` commands. Homebrew has a strong convention for the
naming of `launchd` plists: "homebrew.mxcl.FORMULA_NAME.plist", e.g.
"homebrew.mxcl.postgresql.plist" and "homebrew.mxcl.redis.plist". This likely
makes a function to do the `launchctl` work fairly straightforward.

mac

@@ -64,6 +64,20 @@ brew_is_upgradable() {
 brew_expand_alias() {
   brew info "$1" 2>/dev/null | head -1 | awk '{gsub(/:/, ""); print $1}'
 }
+
+brew_launchctl_restart() {
+  local NAME=$(brew_expand_alias "$1")
+  local DOMAIN="homebrew.mxcl.$NAME"
+  local PLIST="$DOMAIN.plist"
+
+  mkdir -p ~/Library/LaunchAgents
+  ln -sfv /usr/local/opt/$NAME/$PLIST ~/Library/LaunchAgents
+
+  if launchctl list | grep -q $DOMAIN; then
+    launchctl unload ~/Library/LaunchAgents/$PLIST >/dev/null
+  fi
+  launchctl load ~/Library/LaunchAgents/$PLIST >/dev/null
+}
 ### end mac-components/mac-functions
 
 if [[ -f /etc/zshenv ]]; then
@@ -137,13 +151,8 @@ fancy_echo "Installing NVM, Node.js, and NPM, for running apps and installing Ja
   nvm alias default "$node_version"
 ### end mac-components/packages
 
-fancy_echo "Ensure LaunchAgents exists ..."
-  mkdir -p ~/Library/LaunchAgents
-
 fancy_echo "Starting Postgres ..."
-  ln -sfv /usr/local/opt/postgresql/*.plist ~/Library/LaunchAgents
-  launchctl unload ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist >/dev/null
-  launchctl load ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist >/dev/null
+  brew_launchctl_restart postgresql
 ### end mac-components/start-services
 
 fancy_echo "Installing rbenv, to change Ruby versions ..."

mac-components/mac-functions

@@ -26,3 +26,17 @@ brew_is_upgradable() {
 brew_expand_alias() {
   brew info "$1" 2>/dev/null | head -1 | awk '{gsub(/:/, ""); print $1}'
 }
+
+brew_launchctl_restart() {
+  local NAME=$(brew_expand_alias "$1")
+  local DOMAIN="homebrew.mxcl.$NAME"
+  local PLIST="$DOMAIN.plist"
+
+  mkdir -p ~/Library/LaunchAgents
+  ln -sfv /usr/local/opt/$NAME/$PLIST ~/Library/LaunchAgents
+
+  if launchctl list | grep -q $DOMAIN; then
+    launchctl unload ~/Library/LaunchAgents/$PLIST >/dev/null
+  fi
+  launchctl load ~/Library/LaunchAgents/$PLIST >/dev/null
+}

mac-components/start-services

@@ -1,7 +1,2 @@
-fancy_echo "Ensure LaunchAgents exists ..."
-  mkdir -p ~/Library/LaunchAgents
-
 fancy_echo "Starting Postgres ..."
-  ln -sfv /usr/local/opt/postgresql/*.plist ~/Library/LaunchAgents
-  launchctl unload ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist >/dev/null
-  launchctl load ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist >/dev/null
+  brew_launchctl_restart postgresql