ssa_log.c: Add 'accum_log_file' flag

If accum_log_file flag is set, log file (ibssa.log/ibacm.log)
is accumulated across multiple sessions. Otherwise,
the old log file is overwritten and only the new
log is stored.

By default, log file accumulation is off (not used).

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>