measure: separate out PE section options in --help text

measure: separate out PE section options in --help text

Let's separate the PE section options in the --help text to make clearer
that they are about PE sections, and thus make things less confusing
regarding the cryptic names. Specifically, there's now a separate
section in the --help text that looks like this:

<snip>
UKI PE Section Options:                                         UKI PE Section
     --linux=PATH        Path Linux kernel image                → .linux
     --osrel=PATH        Path to os-release file                → .osrel
     --cmdline=PATH      Path to file with kernel command line  → .cmdline
     --initrd=PATH       Path to initrd image                   → .initrd
     --splash=PATH       Path to splash bitmap                  → .splash
     --dtb=PATH          Path to Devicetree file                → .dtb
     --pcrpkey=PATH      Path to public key for PCR signatures  → .pcrpkey
</snipe>

(The section title is underlined, like we usually do it)

I think separating this out, and organizing the section name in tabular
form emphasizes nicely where this switches get their name from and what
they mean.

Hopefully addresses concerns from
https://github.com/systemd/systemd/pull/24458#discussion_r967052720 in a
diferent way.

(Also dropped a hint regarding DER format for public key. it's simply
wrong, we use PEM like everyone else on Linux. Outside of Java land DER
isn't popular, so we don't need to mention it here in the --help text,
man page should be enough.)