Make sure to freezero() the wide-character copy of the user supplied password.
While here, change the mbstowcs() error checking making it complaint with what's
documented in the manual and zap some stray whitespace.

ok benno@ tb@

obvious reallocarray() use for * int overflow; ok doug

Add handling for keyboard layouts which break modhex (like dvorak and many
others); concept and keymap table (covering about 400 layouts) from
modhex.js by Daniel Holth.

ok sthen@, ok halex@ (who also provided much testing and feedback)

make hex and modhex decoding case insensitive, the latter particulary
useful to avoid issues with SHIFT and CAPS LOCK on any keyboard.

pointers on proper tolower usage by guenther@

"Yup" deraadt@, ok sthen@ on initial version
ok guenther@ on final version

remove superfluous RCS identifiers; while here, indent identifier
on Makefile to match style used by other login_* utilities.

ok dhill@

Welcome dhartmei@'s login_yubikey. To be worked on in-tree.

ok millert@
support from mcbride@ and others