complete SO_TIMESTAMPNS to SO_TIMESTAMP fallback

complete SO_TIMESTAMPNS to SO_TIMESTAMP fallback

Commit e8660637ccc3bf2aae163fa7c26216843ae98180 added a
fallback from setting the socket option SO_TIMESTAMPNS to
setting the socket option SO_TIMESTAMP if the nanosecond
timestamp option could not be set.  But it did not add
code to also look for the control message related to
SO_TIMESTAMP.  Thus microsecond timestamps were requested,
but not read.

This commit adds the missing code to read microsecond
timestamp control messages.

The problem was reported in GitHub issue #374 by @payload.