* sysdeps/unix/sysv/linux/Versions [libc, GLIBC_2.4]: Export
unshare.
* sysdeps/unix/sysv/linux/syscalls.list: Add unshare syscall.
* sysdeps/unix/Makefile ($(objpfx)stub-syscalls.c): Add some
preprocessor magic so that the compiler won't see the prototypes
for the functions we are defining as stubs.