Increase MAX_LEN_CAPSTR

[lmoureaux]

Is your feature request related to a problem? Please describe.

Freeciv21 uses capabilities quite heavily to implement backward compatibility. As a result, we're already using one fifth of the space allocated for capstrings in the network protocol, which is only 512 bytes.

Describe the solution you'd like

Instead of relying on MAX_LEN_CAPSTR, make the length dynamic in the protocol.

Describe alternatives you've considered

Increasing MAX_LEN_CAPSTR to some larger static value. It means we'll hit the same problem again in the future (unless we start making capabilities mandatory then removing support for old ones)

Additional context

Adding a cap in #1305