fixes jshttp#165. Previously, validation on serialization used the field-content
regexp from RFC7230 to perform all checks. However, that is the regexp
for the cookie header as a whole, but each individual part of a cookie
has a tighter restriction.

In the bug report jshttp#165 I demonstrated that whitespace in names and values
was invalid but allowed by the field-content pattern. In this code I
started by adding tests for those two cases, then added the expressions
derived from the RFC; the relevant portions of the RFC have been inlined
as comments.

Removing the field-content regexp unearthed that as well as those two
it was being used to validate domain names and paths. Paths are fairly
unrestricted (being any ascii character except control characters and
semicolon) but the code was wrong here too - it allowed all 8-bit characters
not just 7-bit characters.

Domain names have a well recognised pattern, there are only a couple of
gotchas: RFC6265 explicitly says RFC1123 applies, so the leading character
of a label is allowed to be a digit; and while I wondered for a moment
if this should allow things like [::1] the domain matching section of
https://datatracker.ietf.org/doc/html/rfc6265#section-5.1.3
teaches that domain values are _not_ ip addresses.

The regexp should be in the same order as the RFC grammar, for ease
of checking.