Add IPV6_DONTFRAG socket option on OpenBSD

[flub]

• target triple: x86_64-unknown-openbsd
• Header file: https://github.com/openbsd/src/blob/35659bf2453d7eeb12fd24089cf668ca13361c18/sys/netinet6/in6.h#L330C9-L330C24

According to the git history this symbol has been there for a very long time, so there is probably no need to check the openbsd version.

[devnexen]

You re in the luck, seems it is already there.

[flub]

Huh, totally true. I'm afraid I mixed up the symbol, I am looking for IPV6_DONTFRAG:

https://github.com/openbsd/src/blob/35659bf2453d7eeb12fd24089cf668ca13361c18/sys/netinet6/in6.h#L336

Also has been there since forever in OpenBSD, but I don't think is defined by the libc crate.

[tgross35]

This should have been closed by #3716.