Increase EM_MAX_SCATTER to 64 such that the size of em_xmit()::segs[EM_MAX_SCATTER]
doesn't get overrun by things like NFS that can and do shove more than 32 segs when
being used with em(4) and TSO4.

Update tso handling code in em_xmit() with update from jhb@ in email thread:
https://lists.freebsd.org/pipermail/freebsd-net/2014-July/039306.html

set ifp->if_hw_tsomax, ifp->if_hw_tsomaxsegcount & ifp->if_hw_tsomaxsegsize
to appropriate values.

Define a TSO workaround "magic" number of 4 that is used to avoid an
alignment issue in hardware.

Change a couple of integer values that were used as booleans to actual
bool types.

Ensure that em_enable_intr() enables the appropriate mask of interrupts
and not just a hardcoded define of values.

PR: 200221 199174 195078
Differential Revision: https://reviews.freebsd.org/D3192
Reviewed by: erj jhb hiren
MFC after: 2 weeks
Sponsored by: Limelight Networks