dd: add a flag to discard cached data

* src/dd.c (FFS_MASK): A new macro (Find First Set) refactored
from the following enum as it's now used twice.
(usage): Mention the new 'nocache' flag.
(cache_round): A new function to help ignore requests
to drop cache, that are less than page_size.
(invalidate_cache): A new function to call posix_fadvise()
with the appropriate offset and length.  Note we don't
use fdadvise() so we can detect errors when count=0.
(dd_copy): Call invalidate_cache() for the portions read.
(iwrite): Likewise for the portions written.
(main): Call invalidate_cache for page_size slop or
for full file when count=0.
* cfg.mk (sc_dd_O_FLAGS): Adjust to pass.
* doc/coreutils.texi (dd invocation): Describe the 'nocache' flag,
and give some examples of how it can be used.
* tests/dd/nocache: A new test.
* tests/Makefile.am: Reference the new test.
* NEWS: Mention the new feature.

doc/coreutils.texi

@@ -8174,6 +8174,31 @@ last-access and last-modified time) is not necessarily synchronized.
 @cindex synchronized data and metadata I/O
 Use synchronized I/O for both data and metadata.
 
+@item nocache
+@opindex nocache
+@cindex discarding file cache
+Discard the data cache for a file.
+When count=0 all cache is discarded,
+otherwise the cache is dropped for the processed
+portion of the file.  Also when count=0
+failure to discard the cache is diagnosed
+and reflected in the exit status.
+Here as some usage examples:
+
+@example
+# Advise to drop cache for whole file
+dd if=ifile iflag=nocache count=0
+
+# Ensure drop cache for the whole file
+dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0
+
+# Drop cache for part of file
+dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null
+
+# Stream data using just the read-ahead cache
+dd if=ifile of=ofile iflag=nocache oflag=nocache
+@end example
+
 @item nonblock
 @opindex nonblock
 @cindex nonblocking I/O