add library support for tuning TCP_KEEPALIVE

This adds three new options to control the behavior of TCP keepalives:

- CURLOPT_TCP_KEEPALIVE: enable/disable probes
- CURLOPT_TCP_KEEPIDLE: idle time before sending first probe
- CURLOPT_TCP_KEEPINTVL: delay between successive probes

While not all operating systems support the TCP_KEEPIDLE and
TCP_KEEPINTVL knobs, the library will still allow these options to be
set by clients, silently ignoring the values.

include/curl/curl.h

@@ -1499,6 +1499,13 @@ typedef enum {
      of miliseconds. */
   CINIT(ACCEPTTIMEOUT_MS, LONG, 212),
 
+  /* Set TCP keepalive */
+  CINIT(TCP_KEEPALIVE, LONG, 213),
+
+  /* non-universal keepalive knobs (Linux, AIX, HP-UX, more) */
+  CINIT(TCP_KEEPIDLE, LONG, 214),
+  CINIT(TCP_KEEPINTVL, LONG, 215),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;