Deprecate spaces/comments in some regex tokens

Deprecate spaces/comments in some regex tokens

This commit deprecates having space/comments between the first two
characters of regular expression forms '(*VERB:ARG)' and '(?...)'.
That is, the '(' should be immediately be followed by the '*' or '?'.
Previously, things like:

   qr/((?# This is a comment in the middle of a token)?:foo)/

were silently accepted.

The problem is that during regex parsing, the '(' is seen, and the input
pointer advanced, skipping comments and, under /x, white space, without
regard to whether the left parenthesis is part of a bigger token or not.
S_reg() handles the parsing of what comes next in the input, and it
just assumes that the first character it sees is the one that
immediately followed the input parenthesis.

Since the parenthesis may or may not be a part of a bigger token, and
the current structure of handling things, I don't see an elegant way to
fix this.  What I did is flag the single call to S_reg() where this
could be an issue, and have S_reg check for for adjacency if the
parenthesis is part of a bigger token, and if so, warn if not-adjacent.