Filter bad register names

[sanel]

Some modes, like fzf.el, will put :fzf-windows as register name,
causing Evil :register command to fail. This command, but also Vim,
expects registers to be named with number or character.

This is not much fault of fzf.el mode (or similar modes), because
Emacs keeps both window configuration and marks in a single
"register-alist" and mode developers are free to use whetever name for
restoring window configurations.

This commit fixes it by filtering out all register names not conforming to
proper Vim-like naming schema.

[TheBB]

Thanks! Squashed and merged manually.

• I changed the filtering function, since the list is an alist and you want to filter on the keys, not the cons cells.
• I also removed the note in the commit message that fzf.el is not at fault. The documentation of register-alist says that they keys must be numbers, so if some package adds non-numbers there then as far as I am concerned it is their problem. However I guess this filtering doesn't hurt.

[TheBB]

Reported to fzf at bling/fzf.el#54

[sanel]

Thanks!