add missing signal.h include
Add correct exit value to failed tests

(Note from @antirez: modified to apply to 2.8)
Sentinel framework new layout back ported 2.8.

Functionally the same, but makes cherry picking simpler.
redis.tcl: return I/O error message when peer closes connection.
Test: tcl client, unset deferred var on close.
Test: Tcl client initial support for automatic reconnection.
Test: use higher level redis.tcl proc to read replies.