net/fping: Avoid ident collisions

fping derives the ident it puts in the outbound packet to from the
low 16 bits of the process id. This means a busy nagios server can
see collisions. More details on this issue and a patch to solve it
here:

https://github.com/schweikert/fping/issues/206

Apply the patch from the github issue:

PR: 264402
Reported by: Case Larsen
Approved by: jharris (maintainer)