Use standard SIGHUP handler in syslogger.
Commit
1e53fe0e70 changed background processes so that they use
standard SIGHUP handler. Like that, this commit makes syslogger use
standard SIGHUP handler to simplify the code.
Author: Bharath Rupireddy
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/CALj2ACXPorUqePswDtOeM_s82v9RW32E1fYmOPZ5NuE+TWKj_A@mail.gmail.com