provider: Add support for clipboard registers.

This reimplements the '+'/'*' clipboard registers(both are aliases to the same
register, no dedicated storage for the X11 selection) on top of the provider
infrastructure.

This adds two new 'unnamedclip' option, has the same effect of setting
'clipboard' to 'unnamed/unnamedplus' in vim

The 'clipboard' option was not reused because all values(except 'unnamedplus')
seem to be useless for Neovim, and the code to parse the option was relatively
big. The option remains for vim compatibility but it's silently ignored.

src/nvim/normal.c

@@ -923,6 +923,7 @@ getcount:
 
       /* Adjust the register according to 'clipboard', so that when
        * "unnamed" is present it becomes '*' or '+' instead of '"'. */
+      adjust_clipboard_register(&regname);
       set_reg_var(regname);
     }
   }
@@ -5101,6 +5102,7 @@ static void nv_brackets(cmdarg_T *cap)
         end = equalpos(start, VIsual) ? curwin->w_cursor : VIsual;
         curwin->w_cursor = (dir == BACKWARD ? start : end);
       }
+      adjust_clipboard_register(&regname);
       prep_redo_cmd(cap);
       do_put(regname, dir, cap->count1, PUT_FIXINDENT);
       if (was_visual) {
@@ -7267,9 +7269,10 @@ static void nv_put(cmdarg_T *cap)
        */
       was_visual = TRUE;
       regname = cap->oap->regname;
+      bool adjusted = adjust_clipboard_register(&regname);
       if (regname == 0 || regname == '"'
           || VIM_ISDIGIT(regname) || regname == '-'
-
+          || adjusted
           ) {
         /* The delete is going to overwrite the register we want to
          * put, save it first. */