don't croak when the \K follows the lookaround assertion

don't croak when the \K follows the lookaround assertion

this also simplifies the flagging for these assertions, since this
error is now the only thing using in_lookhead and in_lookbehind they
can be combined into a single in_lookaround.

Rather than conditional increment/decrement as we recurse into S_reg
I simply save the value of in_lookaround and restore it before
returning.  Some unsuccessful or restart paths don't do the restore,
but they either result in a croak(), or a restart which reinitialises
in_lookaround anyway.

Also added tests to ensure that all the different zero-width assertions
with content trigger the error.