curl_fnmatch: only allow two asterisks for matching

The previous limit of 5 can still end up in situation that takes a very
long time and consumes a lot of CPU.

If there is still a rare use case for this, a user can provide their own
fnmatch callback for a version that allows a larger set of wildcards.

This commit was triggered by yet another OSS-Fuzz timeout due to this.
Bug: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=8369

Closes #2587