os-posix.c: move code around

this moves code blocks so that functions and variables which
belongs to the same concept are now close to each other.
There's no actual code changes in there.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-ID: <20230901101302.3618955-6-mjt@tls.msk.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>