Add REDIS_MIN_RESERVED_FDS define for open fds

Also update the original REDIS_EVENTLOOP_FDSET_INCR to
include REDIS_MIN_RESERVED_FDS. REDIS_EVENTLOOP_FDSET_INCR
exists to make sure more than (maxclients+RESERVED) entries
are allocated, but we can only guarantee that if we include
the current value of REDIS_MIN_RESERVED_FDS as a minimum
for the INCR size.
Fix potentially incorrect errno usage

errno may be reset by the previous call to redisLog, so capture
the original value for proper error reporting.