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.

lfs.c

@@ -13393,6 +13393,7 @@ static int lfs_deinit(lfs_t *lfs) {
 //
 enum lfsr_rcompat {
     LFSR_RCOMPAT_NONSTANDARD = 0x0001, // Non-standard filesystem format
+    LFSR_RCOMPAT_WRONLY      = 0x0002, // Reading is disallowed
     LFSR_RCOMPAT_GRM         = 0x0004, // May use a global-remove
     LFSR_RCOMPAT_MSPROUT     = 0x0010, // May use an inlined mdir
     LFSR_RCOMPAT_MLEAF       = 0x0020, // May use a single mdir pointer
@@ -13418,6 +13419,7 @@ enum lfsr_rcompat {
 
 enum lfsr_wcompat {
     LFSR_WCOMPAT_NONSTANDARD = 0x0001, // Non-standard filesystem format
+    LFSR_WCOMPAT_RDONLY      = 0x0002, // Writing is disallowed
     // internal
     LFSR_wcompat_OVERFLOW    = 0x8000, // Can't represent all flags
 };