src/socket_wrapper.c: export socket_wrapper_syscall_{valid,va}()

src/socket_wrapper.c: export socket_wrapper_syscall_{valid,va}()

We need to hook into syscall() from socket_wrapper as well as
from uid_wrapper() (and maybe others in future).
But the assumption is that only one wrapper will take care
of a single syscall number.

So we provide socket_wrapper_syscall_valid() in order to allow
external consumers (e.g. uid_wrapper.so) to check if
socket_wrapper wants to handle a specified syscall number.

And we provide socket_wrapper_syscall_va() in order to allow
calling into swrap_syscall().

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>