actions: implement -verbose_action option

Add an action option that prints the result of evaluating
the associated expression, annotating the result of evaluating each
atom (test operation) with =True or =False.

This can be used (especially in debugging action statements) to
show which action statements triggered, and why, and which
action statements didn't trigger and again why.

For instance given the action

-verbose_action 'uid(root)@type(d)||name(file*)'

This could produce the following output

test2/empty=uid(root)@(type(d)=True)
test2/file9=uid(root)@(type(d)=False||name(file*)=True)
test2/file4=uid(root)@(type(d)=False||name(file*)=True)
test2/a=uid(root)@(type(d)=False||name(file*)=False)
test2/sym=uid(root)@(type(d)=False||name(file*)=False)
test2/dir=uid(root)@(type(d)=True)
test2/dir/fred=uid(root)@(type(d)=False||name(file*)=False)
test2/file8=uid(root)@(type(d)=False||name(file*)=True)
test2=uid(root)@(type(d)=True)

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>