e1000: sysctl for TCP flag handling during TSO

Add tso_tcp_flags_mask_first_segment, tso_tcp_flags_mask_middle_segment,
and tso_tcp_flags_mask_last_segment sysctl-variables to control the
handling of TCP flags during TSO.

This allows to change the masks appropriate for classical ECN and to
configure appropriate masks for accurate ECN.

Reviewed by: rrs
MFC after: 3 days
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D44259