Remove a few stray ASIC defines.
core: Remove two unused functions.

They were previously necessary to switch between SHA256d and Scrypt.
core: allow changing TCP keepalive packet idle time using `tcp-keepalive` command-line option or config-file option.

This may be useful in certain scenarios. However, server load from keepalive
is increased 6-fold if code is hard-changed from 30 to 5. So, provide it as
an option instead, and use the previous value as a default (30).

Explanation from

https://github.com/Kalroth/cgminer-3.7.2-kalroth/commit/015c0643961b1355088967ec681a80f2fe427dd1

Kevin's middlecoin fix, CURL TCP keepalive constants lowered:

CURLOPT_TCP_KEEPIDLE from 45 to 5 and CURLOPT_TCP_KEEPINTVL from 30 to
5. Before it'd trigger a keepalive packet after 45 seconds of connection
idle time and then again every 30 seconds. Now it triggers a keepalive
packet after 5 seconds of connection idle time and then again every 5
seconds.

It makes the client more resilient against coin switching pools or just
pools with connection issues in general. It will however add a tiny bit
pressure to the pool server; but a TCP keepalive probe is only about
60-80 bytes, so I don't think it is an issue.