PFSELFTEST improved to test the approximation error.
PFCOUNT: unshare the object when cached cardinality is modified.
PFGETREG added for testing purposes.

The new command allows to get a dump of the registers stored
into an HyperLogLog data structure for testing / debugging purposes.
Remove HyperLogLog type checking duplicated code.
Make hll-gnuplot-graph.rb callable from cli.
Fix PFADD infinite loop.

We need to guarantee that the last bit is 1, otherwise an element may
hash to just zeroes with probability 1/(2^64) and trigger an infinite
loop.

See issue #1657.