recvmmsg(2) is buggy in 11.0-RELEASE, but not recent 11-STABLE or HEAD

• the consequence for rsyslog being that it does not record any log

messages received via UDP to a log file.

recvmmsg(2) fix was applied to HEAD in r307036 and MFC'd to 11-STABLE
in r307311. Therefore prevent configure testing for recvmmsg(2)
before OSVERSION 1100504. Note: this is a no-op for older releases
than 11 -- recvmmsg(2) doesn't exist there.

PR: 214428
Reported by: dch@skunkwerks.at
MFH: 2016Q4