[options] style: enable allow_sticky

Our style will allow to group short options, because we add:
"Style::allow_sticky"

From http://www.boost.org/doc/libs/1_43_0/boost/program_options/cmdline.hpp:
/** Allow to merge several short options together,
    so that "-s -k" become "-sk". All of the options
    but last should accept no parameter. For example, if
    "-s" accept a parameter, then "k" will be taken as
    parameter, not another short option.
    Dos-style short options cannot be sticky.
*/
allow_sticky = short_allow_next << 1,
[options] default value for --config: /etc/boostcached
[options] config: print error only if we can't read "non-default" config

If config is default, we mustn't complain about config not exist, only
if options is passed, and that file didn't exist.