cfitsio: add mirror

Closes #23135.
guile: promote 2.0.9 to stable

We held back on 2.0 as stable way back in #5127 because lilypond didn't
work with it, but now everything in core works with guile 2.0.9.
texxmacs 1.0.7.20
doctor: avoid using MACOS_VERSION
Add OS.mac? and OS.linux?
Rename MacOS to OS::Mac
Move MacOS modules under OS::Mac namespace