fix bad return types for BSDs getdents and debitrot openbsd

[mikdusan]

with this PR:

• fixed getdents signature for freebsd, openbsd, dragonfly
• freebsd 13.2 passes zig test lib/std/std.zig
• openbsd builds stage3

tested on:

• freebsd 13.2, 13.1 (amd64)
• openbsd 7.3 (amd64)
• dragonfly 6.4

impact: I don't foresee an impact on any tier1 platforms, each of the commits are special cased for the respective OS.

other note: Unfortunately I do not have access to a solaris platform to test (and no luck with solaris x86 qemu guest), and getdents return type should be changed from usize → c_int .