BUILD: tcp: condition TCP keepalive settings to platforms providing them

Previous commit b24bc0d ("MINOR: tcp: Support TCP keepalive parameters
customization") broke non-Linux builds as TCP_KEEP{CNT,IDLE,INTVL} are
not necessarily defined elsewhere.

This patch adds the required #ifdefs to condition the visibility of the
keywords, and adds a mention in the doc about their dependency on Linux.
4 files changed