Hard code the default scheme to file

Init no longer changes the default scheme to initfs at any point in
time. And for sandboxing you would be switching scheme namespace, not
default scheme.

It should be possible to mix and match relibc version from before and
after this change without breaking exec, though I haven't tested it.

src/platform/linux/mod.rs

@@ -96,9 +96,6 @@ impl Pal for Sys {
     fn chdir(path: CStr) -> Result<()> {
         e_raw(unsafe { syscall!(CHDIR, path.as_ptr()) }).map(|_| ())
     }
-    fn set_default_scheme(scheme: CStr) -> Result<()> {
-        Err(Errno(EOPNOTSUPP))
-    }
 
     fn chmod(path: CStr, mode: mode_t) -> Result<()> {
         e_raw(unsafe { syscall!(FCHMODAT, AT_FDCWD, path.as_ptr(), mode, 0) }).map(|_| ())