tcp: mechanically rename hostcache metrics structure fields

Use hc_ prefix instead of rmx_. The latter stands for "route metrix" and
is an artifact from the 90-ies, when TCP caching was embedded into the
routing table. The rename should have happened back in 97d8d152c28bb.

No functional change. Done with sed(1) command:

s/rmx_(mtu|ssthresh|rtt|rttvar|cwnd|sendpipe|recvpipe|granularity|expire|q|hits|updates)/hc_\1/g