Fix installation path of header files
Restore accidentally removed G_END_DECLS
Enable TCP keepalives

This commit also adds API to configure keepalive on a InfTcpConnection.
In infinoted, the settings are set such that a keepalive probe is sent after
60s of inactivity, with an interval of 5s between probes. The number of
probes sent before the connection is closed is system-dependent.