Use the MAX_URL_SIZE define where applicable. Increase buffer
sizes for all buffers that need to fit a long pathname - buffers
that need to fit only the hostname (and other short strings, but
not the pathname - such as "headers" in http_connect) are kept
at 1024 bytes for now.

Also increase the max line length in http_read_header, since it
might need to contain a full url for Location: redirects.

Signed-off-by: Martin Storsjö <martin@martin.st>