action: add dircount_range test operation

Compare the number of items in the directory (in the output
filesystem as opposed to the source filesystem) against the supplied
[min, max] value range and return TRUE or FALSE.

Initially believed this and the previous dircount() couldn't
be implemented via my generic TEST_VAR_{RANGE_}FN macros, but in
fact they can.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
GPL 2.0 license file - aka COPYING file

Someones's asked for this - so here it is :)