This makes it possible to fetch from an IPv6 literal without specifying
the -g option.  Globbing remains available elsehwere in the URL.

For example:
  curl http://[::1]/file[1-3].txt



This creates no ambiguity, because there is no overlap between the
syntax of valid globs and valid IPv6 literals.  Globs contain hyphens
and at most 1 colon, while IPv6 literals have no hyphens, and at least 2
colons.

The peek_ipv6() parser simply whitelists a set of characters and counts
colons, because the real validation happens later on.  The character set
includes A-Z, in case someone decides to implement support for scopes
like [fe80::1%25eth0] in the future.

Signed-off-by: Paul Marks <pmarks@google.com>