Adds FreeBSD CMSG helpers and resolve MemberImportVisibility errors

[rafaelcepeda]

Adds FreeBSD CMSG function bindings in BSDSocketAPIPosix.swift and add import CNIOFreeBSD across all NIOPosix files that access struct members defined by FreeBSD system headers.

Motivation:

After wiring CNIOFreeBSD in System.swift, the remaining build errors on FreeBSD were: (1) BSDSocketAPIPosix.swift referencing CNIOLinux_CMSG_* functions that don't exist on FreeBSD, (2) dozens of MemberImportVisibility errors where files access struct members (msg_hdr, msg_len, sin_addr, sa_family, iov_base, etc.) without importing the defining module, and (3) F_SETNOSIGPIPE not existing on FreeBSD.

Modifications:

• Added #elseif os(FreeBSD) case in BSDSocketAPIPosix.swift wiring all 6 CMSG helpers to CNIOFreeBSD_CMSG_* functions.
• Added import CNIOFreeBSD to: BaseSocket.swift, DatagramVectorReadManager.swift, PendingDatagramWritesManager.swift, PendingWritesManager.swift, Socket.swift, SocketChannel.swift.
• Changed ControlMessage.swift to import CNIOFreeBSD instead of CNIOLinux on FreeBSD.
• Added os(FreeBSD) to the global SIGPIPE-ignoring path in SocketProtocols.swift (FreeBSD doesn't have F_SETNOSIGPIPE).

Result:

NIOPosix error count on FreeBSD drops from 75 to 12. Remaining errors are: Vsock support (FreeBSD doesn't have AF_VSOCK), missing in_pktinfo type, and globallyIgnoredSIGPIPE not being defined for FreeBSD.