validator: Conform to the new parser and preprocessor output style

Because I overlooked the validator's dependency on the preprocessor
lineno_string() function, we wound up with badly-formatted output like
the following:

> 20140619 03:16:32 error validation:
> gui/default/widget/slider_minimal.cfg:53
>     included from gui/default.cfg:30
>     included from gui/default/widget/slider_minimal.cfg:100
>     included from gui/default.cfg:30: tag [wrong_tag] may not be used in [resolution]

This commit changes the validator's error formatting functions to
produce output more akin to the parser/preprocessor under the new
scheme introduced in version 1.11.10:

> 20140619 03:17:36 error validation: Tag [wrong_tag] may not be used in [resolution]
> at gui/default/widget/slider_minimal.cfg:53
>     included from gui/default.cfg:30
>     included from gui/default/widget/slider_minimal.cfg:100
>     included from gui/default.cfg:30

There are no translatable strings involved in this change, so this
commit may be safely backported to 1.12.
validator: Remove redundant "on line N" output fragments

They were redundant even before my changes.
validator: Make missing attribute error format more consistent with the rest

Instead of:
> In tag slider_definition which begins here,  missing key id

Use:
> Missing key 'id=' in tag [slider_definition]

Since otherwise the wording is very awkward with the new format (the
file:line line comes after the error), all other key-related messages
use "'key='" to denote attribute names, and there's a double-blank typo
that was there even before I first touched this code.
validator: Update changelog for previous three commits