Work towards wish `keyfile-diagnostic'. Many sshpubk.c keyfile-loading

Work towards wish `keyfile-diagnostic'. Many sshpubk.c keyfile-loading
functions have sprouted `**errorstr' arguments, which if non-NULL can
return a textual error message. The interface additions are patchy and
ad-hoc since this seemed to suit the style of the existing interfaces.

I've since realised that most of this is masked by sanity-checking that
gets done before these functions are called, but it will at least report
MAC failures and the like (tested on Unix), which was the original point
of the exercise.

Note that not everyone who could be using this information is at the
moment.

[originally from svn r3430]