bootctl: reword status/list section headers a bit

bootctl: reword status/list section headers a bit

This is really confusing, let's try to clean this up a bit, in
particular as there are two very similar concepts:

1. The boot loaders, i.e. the category you find systemd-boot, the
   Windows and Apple boot loaders in. These may typically be listed in the
   firmware's EFI variables.

2. The boot loader entries, as defined by the Boot Loader Spec. In this
   category you find the various Linux kernels that are installed, i.e.
   the stuff systemd-boot shows on screen. To make things confusing, the
   Windows and Apple boot loaders can appear both as boot loaders and as
   boot loader entries.

This tries to establish the following nomenclature: "boot loaders" and
"boot loader entries" for these two concepts.