Make the default PID file compile-time settable (#12485)

#12391 offered to change the default PID file from /var/run/gitea.pid however in discussion it was decided that this could break users of older systems. An alternative was offered that we could make the PID file compile/link time settable.

This PR does this, and changes the name of the setting from CustomPID to simply PIDFile. It also updates the from-source docs to show how to change the compiler settings to do this.

Closes #12391

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Florian Klink <flokli@flokli.de>

modules/setting/setting.go

@@ -382,7 +382,8 @@ var (
 	Cfg           *ini.File
 	CustomPath    string // Custom directory path
 	CustomConf    string
-	CustomPID     string
+	PIDFile       = "/var/run/gitea.pid"
+	WritePIDFile  bool
 	ProdMode      bool
 	RunUser       string
 	IsWindows     bool
@@ -535,8 +536,8 @@ func SetCustomPathAndConf(providedCustom, providedConf, providedWorkPath string)
 func NewContext() {
 	Cfg = ini.Empty()
 
-	if len(CustomPID) > 0 {
-		createPIDFile(CustomPID)
+	if WritePIDFile && len(PIDFile) > 0 {
+		createPIDFile(PIDFile)
 	}
 
 	if com.IsFile(CustomConf) {