Modify typedef pcp_flow_t to not hide pointer type
Change of API and inner deeper changes:

    - Add pcp context - returned by pcp_init and used in pcp_new_flow,
      pcp_add_server pcp_terminate and pcp_pulse
    - only one socket used for all pcp servers in one context
    - removed pcp_handle_select, pcp_handle_fd_event, pcp_set_read_fdset,
      pcp_set_fd_change_cb functions
Add table of func pointers for socket operations
Fix crash in a closing of flows with same key bucket
Fix setting of userdata in pcp_new_flow
Fix build issues on WinXP, and OS X
Increase version to 2.0