[Ada] Add some OS constants to control keepalive on TCP connections

[Pierre-Marie de Rodat]

This adds some OS constants that are needed to control the
keepalive status of TCP connections. The new constants are
TCP_KEEPCNT, TCP_KEEPIDLE and TCP_KEEPINTVL.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* s-oscons-tmplt.c: Add some OS constants.
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