Prepare for FreeBSD sandboxing support

This is the utility changes from #9968, which were easier to rebase
first.

I (@Ericson2314) didn't write this code; I just rebased it.

Co-Authored-By: Artemis Tosini <me@artem.ist>
Co-Authored-By: Audrey Dutcher <audrey@rhelmot.io>

src/libutil/linux/include/nix/util/meson.build

@@ -4,5 +4,5 @@ include_dirs += include_directories('../..')
 
 headers += files(
   'cgroup.hh',
-  'namespaces.hh',
+  'linux-namespaces.hh',
 )

src/libutil/linux/linux-namespaces.cc

@@ -1,3 +1,4 @@
+#include "nix/util/linux-namespaces.hh"
 #include "nix/util/current-process.hh"
 #include "nix/util/util.hh"
 #include "nix/util/finally.hh"

src/libutil/linux/meson.build

@@ -1,6 +1,6 @@
 sources += files(
   'cgroup.cc',
-  'namespaces.cc',
+  'linux-namespaces.cc',
 )
 
 subdir('include/nix/util')