Fix FreeBSD's statvfs

[jmdavis]

For some reason, FreeBSD's declarations for core.sys.posix.sys.statvfs are statfs declarations that come from sys/mount.h rather than sys/statvfs.h. This adds core.sys.freebsd.sys.mount and moves the statfs stuff to there and then adds the correct declarations to core.sys.posix.sys.statvfs.

[dlang-bot]

Thanks for your pull request, @jmdavis!

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Testing this PR locally

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dub fetch digger
dub run digger -- build "master + druntime#2265"

[PetarKirov]

I would have grouped the constants in an enum MNT : ulong { /+ ... +/ } aggregate and then "exported" out the individual members as aliases. Easily automatable via static foreach + mixin.

[jmdavis]

Except that we haven't been doing enums like that in druntime. This file does with FFlag, but it's the only one, and I'm actually planning to create a PR after this to remove it so that they're just enums directly in the module like we've done everywhere else when porting constants from C in druntime. While it's not how I'd do it if I were writing proper D code, it's closer to how the C code actually is and thus makes using the C bindings more like using the actual C functions and constants in C.

I also don't see much benefit in having all of the values in there twice, even if the generation of the second set of them is automated.