REORG: check: move tcpchecks away from check.c

REORG: check: move tcpchecks away from check.c

Checks.c remains one of the largest file of the project and it contains
too many things. The tcpchecks code represents half of this file, and
both parts are relatively isolated, so let's move it away into its own
file. We now have tcpcheck.c, tcpcheck{,-t}.h.

Doing so required to export quite a number of functions because check.c
has almost everything made static, which really doesn't help to split!