help: drop a reference to Windows 9x Differential Revision: https://phab.mercurial-scm.org/D7694

help: clarify that the Windows registry key for hgrc files is systemwide Since there's no version or path info here to distinguish between installations, it is effectively systemwide (unless splitting hairs about the WoW64 registry redirection). Differential Revision: https://phab.mercurial-scm.org/D7693

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