BUILD: sockpair: fix build issue on macOS related to variable-length arrays

BUILD: sockpair: fix build issue on macOS related to variable-length arrays

In GH issue #3226, Sergey Fedorov (@barracuda156) reported that since
commit 10c14a1ed0 ("MINOR: proto_sockpair: send_fd_uxst: init iobuf,
cmsghdr, cmsgbuf to zeros"), macOS 10.6.8 with gcc 14.3.0 doesn't build
anymore:

  src/proto_sockpair.c: In function 'send_fd_uxst':
  src/proto_sockpair.c:246:49: error: variable-sized object may not be initialized except with an empty initializer
    246 |         char cmsgbuf[CMSG_SPACE(sizeof(int))] = {0};
        |                                                 ^
  src/proto_sockpair.c:247:45: error: variable-sized object may not be initialized except with an empty initializer
    247 |         char buf[CMSG_SPACE(sizeof(int))] = {0};
        |                                             ^

Upon investigation, it appears that the CMSG_SPACE() macro on this OS
looks too complex for gcc to consider it as a constant, so it takes
these buffers for variable-length arrays and cannot initialize them.

Let's move to a simple memset() instead, which Sergey confirmed fixes
the problem.

This needs to be backported as far as 3.1. Thanks to Sergey for the
report, the bisect and testing the fix.