BUG/MINOR: cli: fix _getsocks with musl libc

BUG/MINOR: cli: fix _getsocks with musl libc

In ticket #1413, the transfer of FDs couldn't correctly work on alpine
linux. After a few tests with musl on another distribution it seems to
be a limitation of this libc.

The number of FD that could be sent per sendmsg was set to 253, which
does not seem to work with musl, decreasing it 252 seems to work
better, so lets set this value everywhere since it does not have that
much impact.

This must be backported in every maintained version.

(cherry picked from commit 148d7a030156ca927e744013dd6fc32b473c1ef6)
Signed-off-by: Willy Tarreau <w@1wt.eu>