handle more cases in sxc_input_fn and add sxc_filter_get_input
fix some compiler warnings
fix Wmissing-field-initializers
Control number of active connections globally and per host.

Default limit for active connections is defined as
MAX_ACTIVE_CONNECTIONS and set to 5.
Default number of active connections with each host is defined as
MAX_ACTIVE_PER_HOST and set to 2.
Both global and per-host limits can be set at runtime.

Add possibility to change those limits in sxcp command with hidden
command line arguments (see sxcp --full-help).