Restore parent mount namespace in restoreProcessContext

This ensures any started processes can't write to /nix/store (except
during builds). This partially reverts 01d07b1e, which happened because
of #2646.

The problem was only happening after nix downloads anything, causing
me to suspect the download thread. The problem turns out to be:
"A  process  can't  join a new mount namespace if it is sharing
filesystem-related attributes with another process", in this case this
process is the curl thread.

Ideally, we might kill it before spawning the shell process, but it's
inside a static variable in the getFileTransfer() function. So
instead, stop it from sharing FS state using unshare(). A strategy
such as the one from #5057 (single-threaded chroot helper binary) is
also very much on the table.

Fixes #4337.

src/libutil/util.hh

@@ -300,7 +300,15 @@ void setStackSize(size_t stackSize);
 
 /* Restore the original inherited Unix process context (such as signal
    masks, stack size, CPU affinity). */
-void restoreProcessContext();
+void restoreProcessContext(bool restoreMounts = true);
+
+/* Save the current mount namespace. Ignored if called more than
+   once. */
+void saveMountNamespace();
+
+/* Restore the mount namespace saved by saveMountNamespace(). Ignored
+   if saveMountNamespace() was never called. */
+void restoreMountNamespace();
 
 
 class ExecError : public Error