keepalives on MacOS X

It looks like the recent keepalives patch won't support MacOS X,
because MacOS X does not have the TCP_KEEPIDLE, TCP_KEEPINTVL, and
TCP_KEEPCNT socket parameters.  It does have this:

#define TCP_KEEPALIVE           0x10    /* idle time used when
SO_KEEPALIVE is enabled */

Should we try to support that as a synonym for TCP_KEEPIDLE, if that's
what it is?  Or not worry about it?  Or... what?

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company