sd-boot: stub: Obtain PE section offsets from RAM, not disk (#6250)

sd-boot: stub: Obtain PE section offsets from RAM, not disk (#6250)

In a Secure Boot scenario the stub loader will have been validated
before execution. A malicious drive could then change the data returned
in future reads, resulting in the loader obtaining incorrect section
offsets and (for instance) allowing the command line to be modified.
Pull that information out of the in-RAM representation of the loader
instead in order to avoid this.

Fixes: #6230

(Lennart did some minor coding style fixes, and renamed pefile.c → pe.c,
as suggested by Kay, given that the file now contains a function whose
name doesn't match the filename as prefix anymore.)