Problem:    'wildmenu' only shows few matches.
Solution:   Add the "pum" option: use a popup menu to show the matches.
            (Yegappan Lakshmanan et al., closes vim/vim#9707)
https://github.com/vim/vim/commit/3908ef5017a6b4425727013588f72cc7343199b9

Omit p_wmnu check in cmdline_pum_active() as it can cause problems.
Omit vim_strchr() flags as that isn't really better than bitmasks.
Omit key translations and document it in vim_diff.txt.