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).
Do not accept negative connections limits for sxcp
Connections limiting cosmetics.

Fix sxcp help page indentation.
Fix indentation
Fix ev_fifo_add() error checking.
Return NULL when sxi_ht_new() fails in connection_pool_new()