Set the variable "prog" before the first call to any routine using error reporting

Set the variable "prog" before the first call to any routine using error reporting

Error reporting uses the variable "prog" which is null prior to obtaining a
reference to argv[0]. The call to open_ping_socket() uses this on an error,
such as the executable not being SUID or otherwise privileged invocation.
Solaris 10 printf(3C) of "%s" on a null pointer causes a core dump.