bin/xbps-alternatives: add repository mode to list repo alternatives

[Duncaen]

No description provided.

[Duncaen]

xbps-alternatives -Rl produces https://gist.github.com/Duncaen/578435c3619c372ee2463ef79dbd5edb.

This also works for specific groups:

$ ./bin/xbps-alternatives/xbps-alternatives -Rlg vim
vim
 - gvim-8.2.1666_2
  - vim:/usr/bin/gvim
 - gvim-huge-8.2.1666_2
  - vim:/usr/bin/gvim-huge
 - neovim-0.4.4_1
  - vim:/usr/bin/nvim
  - vim.1:/usr/share/man/man1/nvim.1
 - vim-8.2.1666_2
  - vim:/usr/bin/vim-normal
 - vim-huge-8.2.1666_2
  - vim:/usr/bin/vim-huge
 - vim-x11-8.2.1666_2
  - vim:/usr/bin/vim-x11
$ # without -R
$ ./bin/xbps-alternatives/xbps-alternatives -lg vim
vim
 - vim (current)
  - vim:/usr/bin/vim-normal

[lgtm-com]

This pull request introduces 1 alert when merging 8052917 into 05ff04a - view on LGTM.com

new alerts:

• 1 for Empty branch of conditional

[classabbyamp]

this looks like it would fix my issue (#499), but it just exits without doing anything:

$ LD_LIBRARY_PATH=./lib/ bin/xbps-alternatives/xbps-alternatives -dRlg vim 
[DEBUG] XBPS: 0.60 API: 20200423 GIT: a9c3af43
[DEBUG] Processing configuration directory: /usr/local/etc/xbps.d
[DEBUG] Processing system configuration directory: /usr/local/share/xbps.d