`%pastebin` arguments get silently dropped if they are out of order

%pastebin 1 -d "This is a test"

Intuitively, the above should result in either:

• An error, since -d "This is a test" should come first.
• A dpaste.org entry where the description is set to "This is a test".

Instead, it silently drops the arguments.

• Find which builtin magics do this sort of thing (Magics.parse_options is used for the arguments in %pastebin - look for the ones that use it).
• Fix them - preferably by enabling unordered arguments, whenever possible.

Found out during the review of #13056.