Handle pcfiledir variable like normal variable

Rather than responding to the pcfiledir variable specially, store it
in the hash table with all the other package specific variables. This
does result in pcfiledir showing up in the --print-variables output.
check: Test that the pcfiledir variable is output correctly
Quote pc_path virtual variable

Since we unquote variables on printing, we want to quote in before adding
to the database. For all other variables defined in .pc files, we assume
that the user has quoted the variable correctly.

This was broken with the fix for bug 67904, but seems to only affect
Windows builds.