MFC r340245: ping(8): improve diagnostics in case of wrong arguments.

MFC r340245: ping(8): improve diagnostics in case of wrong arguments.

  For example, in case of super-user:
  $ sudo ping -s -64 127.0.0.1
  PING 127.0.0.1 (127.0.0.1): -64 data bytes
  ping: sendto: Invalid argument

  For unprivileged user:
  $ ping -s -64 127.0.0.1
  ping: packet size too large: 18446744073709551552 > 56: Operation not permitted

  Fix this by switching from strtoul() to strtol() for integer arguments
  and adding explicit checks for negative values.