--pidfile parameter for the dragonflydb binary

[anosulchik]

--pidfile=</path/to/dragonflybd.pid> would make it easier to integrate it with monit. When this param is specified, dragonflydb would write its pid into specified file or fail if it's possible e.g. due to permission.

[iko1]

@romange, I'd like to implement a function that creates pidfilefile, where is the suitable place to place that function? I think the file proc_util.c within helio can be an option. The function will be called from server bootstrap in dfly_main.cc

[romange]

Thanks @iko1 . Let's just implement everything inside dfly_main.cc.