extmod/vfs_fat_file: Implement SEEK_CUR for non-zero offset.

CPython doesn't allow SEEK_CUR with non-zero offset for files in text mode,
and uPy inherited this behaviour for both text and binary files.  It makes
sense to provide full support for SEEK_CUR of binary-mode files in uPy, and
to do this in a minimal way means also allowing to use SEEK_CUR with
non-zero offsets on text-mode files.  That seems to be a fair compromise.

extmod/vfs_fat_file.c

@@ -134,11 +134,7 @@ STATIC mp_uint_t file_obj_ioctl(mp_obj_t o_in, mp_uint_t request, uintptr_t arg,
                 break;
 
             case 1: // SEEK_CUR
-                if (s->offset != 0) {
-                    *errcode = MP_EOPNOTSUPP;
-                    return MP_STREAM_ERROR;
-                }
-                // no-operation
+                f_lseek(&self->fp, f_tell(&self->fp) + s->offset);
                 break;
 
             case 2: // SEEK_END

tests/extmod/vfs_fat_fileio1.py

@@ -91,10 +91,8 @@ with open("foo_file.txt") as f2:
 
     f2.seek(0, 1) # SEEK_CUR
     print(f2.read(1))
-    try:
-        f2.seek(1, 1) # SEEK_END
-    except OSError as e:
-        print(e.args[0] == uerrno.EOPNOTSUPP)
+    f2.seek(2, 1) # SEEK_CUR
+    print(f2.read(1))
 
     f2.seek(-2, 2) # SEEK_END
     print(f2.read(1))

tests/extmod/vfs_fat_fileio1.py.exp

@@ -7,7 +7,7 @@ hello!world!
 12
 h
 e
-True
+o
 d
 True
 [('foo_dir', 16384, 0)]