windows: add a global equivalent to /etc/mercurial for *.rc processing This follows the Unix model of processing this directory immediately after <internal>/*.rc, and prior to the installation relative files. Since the Unix processing supports both a directory and a file (the former overriding the latter), and since %HOME% supports both `*.ini` and `.hgrc` (again, the former overriding the latter), this does too. The Unix file doesn't have a `.` prefix, so it's not used here either. Note that this is the opposite order of processing the exe relative paths. But since it's in agreement with Unix, %HOME% and %USERPROFILE%, it seems reasonable to ignore that. Maybe we can change that and take a BC, because that's something the installer should be controlling, and I can't imagine people having both paths *and* conflicting settings. Differential Revision: https://phab.mercurial-scm.org/D7692