ssa.c: Code Refactoring. Use define rather than number for upstream fds number.
ssa.c,common.h: Code Refactoring. Rename: timeout -> umad_timeout
common.h ssa.c: Add join timer.
acm, distrib: Add rejoin_timeout configuration parameter.

 rejoin_timeout
 Specifies start timeout (in sec.) before first rejoin attempt.
 Exponential backoff is used to space out join retries.
ssa.c: Avoid to start join timer if rejoin_timeout < 0.