Test: check that replication partial sync works if we break the link.

The test checks both successful syncs and unsuccessful ones by changing
the backlog size.
Version bumped to 2.9.10
Test: more PSYNC tests (backlog TTL).
Obtain absoute path of configuration file, expose it in INFO.
CONFIG REWRITE: Initial support code and design.
CONFIG REWRITE: support for save and syslog-facility.
CONFIG REWRITE: support for dir and slaveof.
CONFIG REWRITE: support for client-output-buffer-limit.
CONFIG REWRITE: strip multiple empty lines.
CONFIG REWRITE: remove orphaned lines.
redis-cli: help.h updated.
CONFIG REWRITE: actually rewrite the config file, atomically.
CONFIG REWRITE: Use sane perms when creating redis.conf from scratch.
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.