Inconsistent behavior for directional rose

*Originally reported in https://github.com/GenericMappingTools/pygmt/pull/4025#issuecomment-3673413020.*

The issues are:

- Directional roses are plotted at bottom-left (`x0/0`) by default, but it seems the anchor point is "BL" for a simple rose and "MC" for a fancy rose. So, the behavior is inconsistent.
- The box is not drawn for the simple rose when the position is `x0/0`
- Setting position to jBL looks good, but the box for the simple rose seems to be a square which doesn't fit the simple rose well.

```bash
gmt begin map png
gmt basemap -R100/110/30/35 -JM10c -Baf
gmt basemap -Td+w2c -F+pblue
gmt basemap -Td+w2c+f -F+pred

gmt basemap -R100/110/30/35 -JM10c -Baf -Xw+2c
gmt basemap -Tdx0/0+w2c -F+pblue
gmt basemap -Tdx0/0+w2c+f -F+pred

gmt basemap -R100/110/30/35 -JM10c -Baf -Xw+2c
gmt basemap -TdjBL+w2c -F+pblue
gmt basemap -TdjBL+w2c+f -F+pred
gmt end show
```

Another issue is the default width. The GMT docs say the default width is 10%, but the following script crashes
```
gmt begin map png
gmt basemap -R100/110/30/35 -JM10c -Baf
gmt basemap -Tdx0/0 -F+pblue
gmt basemap -Tdx0/0+f -F+pred
gmt end show
```
The error message is:
```
basemap [ERROR]: Option -T: Correct syntax
basemap [ERROR]: -T[f|m][x]//[/][:wesnlabels:][+[/]]
basemap [ERROR]: Could not decode dx0, return NaN.
basemap [ERROR]: Option -T: Could not parse geographic coordinates dx0 and/or 0
basemap [WARNING]: not a valid number and may not be decoded properly.
basemap [ERROR]: Option -T: Size must be positive
basemap [ERROR]: Option -T: must be 1, 2, or 3
```
The default width works well if position is not `x0/0`. I think it's because when `x0/0` is used, GMT fallback to the old GMT syntax, which doesn't have default width set.