clipboard: clean up "unnamedclip" logic
clipboard: avoid copying from/to unnamed, use it directly (when unnamedclip set)
clipboard: let provider get/set register type (character/linewise)
vroom test for clipboard provider
change option "unnamedclip" back to "clipboard=unnamed/unnamedplus"

This allows to configure which of '*' and '+' should be used for
the unnamed clipboard