[perl #97958] Make reset "" match its docs

[perl #97958] Make reset "" match its docs

According to the documentation, reset() with no argument resets pat-
terns.  But reset "" and reset "\0foo" were also resetting patterns.
While I was at it, I fixed embedded nulls, too, though it’s not likely
anyone is using this.  I could not fix the bug within the existing API
for sv_reset, so I created a new function and left the old one with
the old behaviour.  Call me pear-annoyed.