fix: add MAP_SHARED to map_mut_anywhere flags

validate_kfmap_flags in kernel proc scheme requires exactly one of
MAP_SHARED or MAP_PRIVATE. Without it, shared == private == false,
and the check fails with EINVAL. This was the root cause of the
fexec_impl crash at step 62 (first PT_LOAD segment mapping).

redox-rt/src/proc.rs

@@ -740,7 +740,7 @@ impl<'a> MmapGuard<'a> {
                 size,
                 offset,
                 address: 0,
-                flags: PROT_READ | PROT_WRITE,
+                flags: PROT_READ | PROT_WRITE | MapFlags::MAP_SHARED,
             },
         )?;
         let slice = unsafe { &mut *this.as_mut_ptr_slice() };