Set correct context node when initializing xsl:keys

This makes the current() function in "use" expressions work properly.
Fixes bugs #530871, #607893, and #676335.
Make --maxvars option work
Helper functions to evaluate compiled XPath expressions

Reduces code size in libxslt/transform.c and makes the code more
readable.
Set namespaces before evaluating xsl:number

Fixes bug #712173.