adding some smartness in file cache

[Wojciech Puchar]

FreeBSD uses unified cache which is generally great but have some 
disadventages.

One common is case of linear reading or writing of large files.
This wipes out other things from memory very quickly.

Can cache be made to drop every full block (MAXPHYS) from cache instantly 
after is fully read by user program or  is wrote to disk.

This way memory would be much better utilized in caching small I/O.