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/libstore/filetransfer.cc

@@ -14,7 +14,7 @@
 #endif
 
 #ifdef __linux__
-# include "nix/util/namespaces.hh"
+# include "nix/util/linux-namespaces.hh"
 #endif
 
 #include <unistd.h>

src/libstore/globals.cc

@@ -85,7 +85,7 @@ Settings::Settings()
         builders = concatStringsSep("\n", ss);
     }
 
-#if defined(__linux__) && defined(SANDBOX_SHELL)
+#if (defined(__linux__) || defined(__FreeBSD__)) && defined(SANDBOX_SHELL)
     sandboxPaths = tokenizeString<StringSet>("/bin/sh=" SANDBOX_SHELL);
 #endif
 

src/libstore/include/nix/store/globals.hh

@@ -682,7 +682,9 @@ public:
             description of the `size` option of `tmpfs` in mount(8). The default
             is `50%`.
         )"};
+#endif
 
+#if defined(__linux__) || defined(__FreeBSD__)
     Setting<Path> sandboxBuildDir{this, "/build", "sandbox-build-dir",
         R"(
             *Linux only*

src/libstore/unix/build/linux-derivation-builder.cc

@@ -2,7 +2,7 @@
 
 #  include "nix/store/personality.hh"
 #  include "nix/util/cgroup.hh"
-#  include "nix/util/namespaces.hh"
+#  include "nix/util/linux-namespaces.hh"
 #  include "linux/fchmodat2-compat.hh"
 
 #  include <sys/ioctl.h>

src/libstore/unix/user-lock.cc

@@ -197,7 +197,7 @@ bool useBuildUsers()
     #ifdef __linux__
     static bool b = (settings.buildUsersGroup != "" || settings.autoAllocateUids) && isRootUser();
     return b;
-    #elif defined(__APPLE__)
+    #elif defined(__APPLE__) && defined(__FreeBSD__)
     static bool b = settings.buildUsersGroup != "" && isRootUser();
     return b;
     #else