Added a define for most configuration defaults.

Also the logfile option was modified to always have an explicit value
and to log to stdout when an empty string is used as log file.

Previously there was special handling of the string "stdout" that set
the logfile to NULL, this always required some special handling.
Test default config modified to set logfile to empty string.

Required because of recent changes in the way logfile is set to standard
output.
CONFIG REWRITE: repl-disable-tcp-nodelay is a boolean option.
CONFIG REWRITE: fixed typo in AOF fsync policy.
CONFIG REWRITE: "active-rehashing" -> "activerehashing".
CONFIG REWRITE: correctly escape the notify-keyspace-events option.
CONFIG REWRITE: when rewriting amount of bytes use GB, MB, KB if possible.
CONFIG REWRITE: bindaddr -> bind.
CONFIG REWRITE: backlog size is a bytes option.
Use memtoll() when parsing the backlog-size option.