Wildcard fix for file-engine

The current implementation (D5-branch) of caching using the FILE-engine does not provide a solution for wildcards in cache_clear_all();

Problem:
All $keys are stored as there md5-counterpart:

ex: content:160137:160831  
=> stored as fffe5d13a598d9952eafc0bf1b64ba7d  

This way we can not find any 'related' keys when cache_clear_all() request the deletion of "content:160137*"

Possible solution:
we should add some more information in the filename

ex: content:160137:160831  
=> stored as fffe5d13a598d9952eafc0bf1b64ba7d--content:160137:160831

file_scan_directory() can be used to find all cache-files that fall into the group "content:160137*".

This issue is related to #279152

My patch is in attachment. It adds the current $key to the end of the md5 cache-filename; and it searches for all the files when cache_clear_all() is called with wirldcard=TRUE.