Bug 3845: http_port tcpkeepalive= option fails parsing

Bug 3845: http_port tcpkeepalive= option fails parsing

The stricter xato*() parsing bounds checks are halting on the ','
delimiters. Fix this by adding an optional end-of-value parameter to the
relevant parse functions and sending the delimiter in.

This fix makes xatoui() and xatoll() more friendly to parsing
unterminated strings.

previous to this fix the latter two values of tcpkeepalive= were
undocumented optional. This makes Squid enforce the documented format
where all three values are required if any is set.