tcp: improve compilation of cc and their helper modules

[tuexen]

Summary

• Allow the congestion control modules CDG, CHD, HD, and vegas also to be compiled into the kernel for IPv6 only kernel configs
• Compile the helper module h_ertt for round trip time estimation only into the kernel, if one of the congestion control module using it (one of CDG, CDH, HD, or vegas) is also compiled into the kernel.

Please note the second point implies that TCP TSO is done whenever possible as long as none of the congestion control modules CDG, CHD, HD, or vegas is loaded or compiled into the kernel.

[timo.voelker_fh-muenster.de]

The suggested changes in the summary make sense to me and the changes in sys/conf/files seem to correctly correspond to them. Looks all good to me. However, I haven't touched this file and don't know enough to review it.