Converting between fill argument and fill structure

In dealing with issue #3696 I found another problem in how the fill syntax works. In the **coast** version tested in #3696 I build input for **psxy** to plot DCW polygons instead of doing it polygon by polygon because of the hole issue. It fails for ex34 because I parse the pattern argument +gP300/8 and it gets stored in a GMT_FILL struct. However, when I want to do the reverse (write what the structure holds back to a string) it gives me p300/8; notice the lower-case **p**. Per our documentation

> By specifying upper case -GP instead of -Gp the image will be bit-reversed

so while since the p is now P we lost that bit of information.

Internally, when p300/8 is parsed we check for p and if it is p we bit-reverse the image. This seems to contradict the documentation but that might just be an internal thing. However, if that is true then we should write P300/8 on the output, not p. So this is a bit messy but it is clearly a bug that would never be found without the DCW issue! I note that once the image is bit-reversed there is no information left that this occurred. All we know is it is pattern number 8.