Squash of LFS_READONLY cleanup

- undef unavailable function declarations altogether
- even less code, assert on write attempts
- remove LFS_O_WRONLY and other flags when compiling with LFS_READONLY
- do not annotate #endif, as requested
- move ifdef before comments blocks, rework dangling opening bracket
- ifdef file flags that are not needed in read-only mode
- slight refactor
- ifdef LFS_F_ERRED out as well

lfs.h

@@ -77,7 +77,6 @@ enum lfs_error {
     LFS_ERR_EXIST       = -17,  // Entry already exists
     LFS_ERR_NOTDIR      = -20,  // Entry is not a dir
     LFS_ERR_ISDIR       = -21,  // Entry is a dir
-    LFS_ERR_NOSYS       = -38,  // Function not implemened
     LFS_ERR_NOTEMPTY    = -39,  // Dir is not empty
     LFS_ERR_BADF        = -9,   // Bad file number
     LFS_ERR_FBIG        = -27,  // File too large