qtaguid.cpp - improvements

Resolves a pair of TODO's, and makes a pair of error return
code paths not return null function pointers.

Note that:
  system/netd/client/NetdClient.cpp
implements this as:

int checkSocket(int socketFd) {
    if (socketFd < 0) {
        return -EBADF;
    }
    int family;
    socklen_t familyLen = sizeof(family);
    if (getsockopt(socketFd, SOL_SOCKET, SO_DOMAIN, &family, &familyLen) == -1) {
        return -errno;
    }
    if (!FwmarkClient::shouldSetFwmark(family)) {
        return -EAFNOSUPPORT;
    }
    return 0;
}

$define CHECK_SOCKET_IS_MARKABLE(sock) \
    do {                               \
        int err = checkSocket(sock);   \
        if (err) return err;           \
    } while (false)

extern "C" int tagSocket(int socketFd, uint32_t tag, uid_t uid) {
    CHECK_SOCKET_IS_MARKABLE(socketFd);
    FwmarkCommand command = {FwmarkCommand::TAG_SOCKET, 0, uid, tag};
    return FwmarkClient().send(&command, socketFd, nullptr);
}

extern "C" int untagSocket(int socketFd) {
    CHECK_SOCKET_IS_MARKABLE(socketFd);
    FwmarkCommand command = {FwmarkCommand::UNTAG_SOCKET, 0, 0, 0};
    return FwmarkClient().send(&command, socketFd, nullptr);
}

which means it *already* verifies that the passed in sockfd
is >= 0 and a socket via getsockopt(SOL_SOCKET, SO_DOMAIN),
as such the 'fcntl(sockfd, F_GETFD)' check is spurious.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I91ef68be5b0cc6b1972d514c13a76eaf834a3d5d