CreateListener creates a listener from inherited fd if there is no inherited fd, create a now one.

CreateUDPConn creates a udp connection from inherited fd if there is no inherited fd, create a now one.

GetAllLisenFiles returns all listen files.

GraceHandler set the signle handler for grace restart.

GraceHandler send signal USR2 to pid.

InheritFdPrefix marks the fd inherited from parent process.