Added LFS_WCOMPAT_RDONLY and LFS_RCOMPAT_WRONLY

LFS_WCOMPAT_RDONLY seems generally useful for tools that just want to
mark a filesystem is read-only. This is a common flag that exists in
other filesystems (RO_COMPAT_READONLY in ext4 for example).

LFS_RCOMPAT_WRONLY, on the other hand, is a bit more of a joke, but
there could be some niche use cases for it (preventing double mounts?).

Fortunately, these flags require no extra code, and fall out naturally
from our wcompat/rcompat handling.

---

Originally, the idea was to also add LFS_F_RDONLY, to match LFS_M_RDONLY
and set the LFS_WCOMPAT_RDONLY flag during format.

But this doesn't really work with the current API, since lfsr_format
would just give you an empty filesystem you can't write to. Which is a
bit silly.

Maybe we should add something like lfsr_fs_mkrdonly in the future? This
is probably low-priority.

scripts/dbgflags.py

@@ -149,6 +149,7 @@ FLAGS = [
     # Read-compat flags
     ('RCOMPAT', 'NONSTANDARD',
                            0x0001, "Non-standard filesystem format"           ),
+    ('RCOMPAT', 'WRONLY',  0x0002, "Reading is disallowed"                    ),
     ('RCOMPAT', 'GRM',     0x0004, "May use a global-remove"                  ),
     ('RCOMPAT', 'MSPROUT', 0x0010, "May use an inlined mdir"                  ),
     ('RCOMPAT', 'MLEAF',   0x0020, "May use a single mdir pointer"            ),
@@ -164,6 +165,7 @@ FLAGS = [
     # Write-compat flags
     ('WCOMPAT', 'NONSTANDARD',
                            0x0001, "Non-standard filesystem format"           ),
+    ('WCOMPAT', 'RDONLY',  0x0002, "Writing is disallowed"                    ),
 
     ('wcompat', 'OVERFLOW',0x8000, "Can't represent all flags"                ),