libpmcstat: Extract parts of pmcstat_image_get_elf_params

[jrtc27]

Summary

These parts of the function will also be needed when parsing separate
debug files, so extract them to new internal functions that can be
reused when doing so in a future commit.

As part of this, fix up all the warning messages to use the right path
when fd refers to FSROOT/boot/modules/NAME; previously it always used
the FSROOT/KERNELPATH/NAME string which could get confusing. Also fix a
style issue for the done label.

MFC after: 1 month

[jrtc27]

Initialise minva/maxva in the right place

[jhb]

lib/libpmcstat/libpmcstat_image.c

427

Why are we not exporting the full path for kernel modules? The kernel linker stores the full path of the module (and has for many years now). We should probably fix that (and since you are bumping the major version of PMC maybe go ahead and fix that as part of this series?)

[jrtc27]

lib/libpmcstat/libpmcstat_image.c

427

Can that be relied upon, or is pathname sometimes still not full? Otherwise looks straightforward, just two places to switch I believe.

[jhb]

lib/libpmcstat/libpmcstat_image.c

427

kgdb has relied upon it for over a decade I think (maybe 15 years?)

16 years actually: https://github.com/freebsd/freebsd-src/commit/1676805c186f5dea36d331a982480d7d9693b126

hwpmc just predates this change I think and was never updated to catch up