iochannel: don't use variable length array in union

iochannel: don't use variable length array in union

Clang didn't like the variable length array:

pulsecore/iochannel.c:358:17: error: fields must have a constant size:
'variable length array in structure' extension will never be supported
        uint8_t data[CMSG_SPACE(sizeof(int) * nfd)];
                ^

Commit 451d1d6762 introduced the variable length array in order to have
the correct value in msg_controllen. This patch reverts that commit and
uses a different way to achieve the same goal.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=99458