Release v2.9

This release make the --rename mode work the same as the default --move, since we can now rename directories freely in any case. The --rename flag is no longer documented, but is still accepted by mmv for backwards compatibility.

Some code that previously tried to drop privileges for most but not all uses was removed, as it served no useful purpose, and triggered static analysis tools that diagnosed a security flaw (setuid without setgroups). mmv is really just a fancy version of mv/cp/ln, which do not drop privileges; this behaviour was neither expected nor desired.