Use posix_spawn to run the pager

In low memory environments, "nix-env -qa" failed because the fork to
run the pager hit the kernel's overcommit limits. Using posix_spawn
gets around this. (Actually, you have to use posix_spawn with the
undocumented POSIX_SPAWN_USEVFORK flag, otherwise it just uses
fork/exec...)

src/libutil/types.hh

@@ -73,6 +73,7 @@ class SysError : public Error
 public:
     int errNo;
     SysError(const FormatOrString & fs);
+    SysError(int errNo, const FormatOrString & fs);
 };