kernel - Adjust ssb_space_prealloc() use cases

kernel - Adjust ssb_space_prealloc() use cases

* Add two flags to the signalsockbuf ssb_flags field.

  SSB_PREALLOC  - Indicates that data preallocation tracking is being used
  SSB_STOPSUPP	- Indicates that SSB_STOP flow control is being used

* unix domain sockets set SSB_STOPSUPP, tcp and sctp sockets
  set SSB_PREALLOC.

* sendfile() requires that either SSB_PREALLOC or SSB_STOPSUPP be specified.

* Code now conditionalizes the use of ssb_space() vs ssb_space_prealloc()
  based on the presence of the SSB_PREALLOC flag.

Reported-by: sephe