server: write main daemon PID to run directory

so that we can easily get the main PID of the last recently launched
daemon. Will be used to get the control socket of that one for access
lgo rotate in a future patch

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/bin/proxmox-backup-api.rs

@@ -78,6 +78,7 @@ async fn run() -> Result<(), Error> {
         },
     );
 
+    server::write_pid(buildcfg::PROXMOX_BACKUP_API_PID_FN)?;
     daemon::systemd_notify(daemon::SystemdNotify::Ready)?;
 
     let init_result: Result<(), Error> = try_block!({