Significant performance improvement for mmap'ed files.  This commit makes MADV_SEQUENTIAL much more effective.  I suggest that we start using MADV_SEQUENTIAL on system utilities that mmap their input files, and the I/O is predominantely sequential. Below is a test with 'cmp' on two relatively large binary files, where the files are so large that the caching is ineffective:
Significant performance improvement for mmap'ed files.  This commit
makes MADV_SEQUENTIAL much more effective.  I suggest that
we start using MADV_SEQUENTIAL on system utilities that mmap
their input files, and the I/O is predominantely sequential.
Below is a test with 'cmp' on two relatively large binary files,
where the files are so large that the caching is ineffective:

+ ls -l t1.xxx t2.xxx
-rw-r--r--  1 root  wheel  65598384 Dec 10 12:13 t1.xxx
-rw-r--r--  1 root  wheel  65598384 Dec 10 12:14 t2.xxx

+ time cmp t1.xxx t2.xxx
3.78user 0.70system 1:33.43elapsed 4%CPU

+ time cmpmadv t1.xxx t2.xxx
4.21user 1.05system 0:30.93elapsed 17%CPU

This change is as a result of an observation made by BDE.

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