Validate Content-Length value prefix (#629)

Validate Content-Length value prefix (#629)

The new code detects all invalid Content-Length prefixes but the old
code was already rejecting most invalid prefixes using strtoll(). The
newly covered (and now rejected) invalid characters are

* explicit "+" sign;
* explicit "-" sign in "-0" values;
* isspace(3) characters that are not (relaxed) OWS characters.

In most deployment environments, the last set is probably empty because
the relaxed OWS set has all the POSIX/C isspace(3) characters but the
new line, and the new line is unlikely to sneak in past other checks.

Thank you, Amit Klein <amit.klein@safebreach.com>, for elevating the
importance of this 2016 TODO (added in commit a1b9ec2).