add option to force cachestat() usage

add option to force cachestat() usage

When cachestat() is not available, mmap() and fincore() are used to get
cache statistics.
This fallback returns less data than cachestat, and potentially hides
read errors by reporting that the file is all mapped in memory:

	$ fincore /usr/bin/java
	fincore: failed to do cachestat: /usr/bin/java: Operation not permitted
	  RES PAGES   SIZE FILE
	 208K    13 198,1K /usr/bin/java
	$ echo $?
	0
	$ sudo fincore /usr/bin/java
	RES PAGES   SIZE FILE
	 0B     0 198,1K /usr/bin/java

Add a --cachestat argument which forces cachestat usage and fails if
it's not available:

	$ fincore -C /usr/bin/java
	fincore: failed to do cachestat: /usr/bin/java: Operation not permitted
	$ echo $?
	1

Also, on huge files the fallback is very slow and memory consuming,
so this flag can be user to avoid doing it by mistake.