add new gzip-rsyncable compression method

This adds a new method = gzip-rsyncable option for plugins that
support the holland.lib.compression api.  This will run
gzip --rsyncable to provide somewhat better efficiency when
using rsync to maintain backup files.
handle stream = bool options more compatibly

Initially the xtrabackup changes in 1.0.8 only
accept stream = (yes,no,tar,xbstream), but this patch
ensures that we handle the various boolean options the
old v1.0.6 config format allowed so backwards compatibiity
is preserve across minor releases.
Merge branch 'gzip_rsyncable'
fix serious bug where streaming backps redicted to /dev/null

When determining the filename where a backup should be stored,
the xtrabackup plugin only considered values for stream of 'tar'
and 'xbstream', so when using the 1.0.6 default value of
stream = yes, this would generate an empty backup.
This patch ensures we're always comparing the stream value
through determine_stream-method() so we have a consistent
view of the real backup method request, where boolean true
values map to 'tar' and false values map to a non-streaming
method.
Merge branch 'xtrabackup_compat'
add import shlex import
Merge branch 'gzip_rsyncable'
format compression format properly in mysqldump plugin
ensure config validation method 'force_list' treats empty strings as empty lists

Previously a list accepting option that was set to an empty string would
generate a list with a single empty-string vaue.  This would cause
confusing errors and require additional checks to work around.

This patch ensures force_list will return an empty list of a value is
a false value. so "option = " is equivalent to "option = ,"