Fix sdcard fragmentation issues caused by cluster gaps between pre-populated files

[gurcei]

A sibling card to mega65-core's ticket: https://github.com/MEGA65/mega65-core/issues/670

Most of the details are on that card.

[gurcei]

Just to share more of the backstory of the work done on this ticket:

• I made repairs to assure the make m65fdisk worked again (for the linux version of the fdisk tool)
• The behaviour of this linux m65fdisk tool's input arguments has changed
  • Previously, it was hard-coded to always assume that the sd-card was "/dev/sdb" (which was a little dangerous)
  • Now, you can now specify two input env-vars:
  • export SDCARDFILE=sdcard.img (or could point to your sd-card device like /dev/sdb if you prefer)
  • export FLASHFILE=/path/to/your/mega65r3.cor (this is needed to test behaviour of pre-populating files)
• With linux m65fdisk now working, this allowed me to add in a test within gtest/m65fdisk_test.cpp that is running fine on the jenkins builder
• Inside fdisk.c, I broke out parts of the overly lengthy main() function into separate functions, to help with readability of top-level behaviour.

PS. I did give make format a run, but it touched too many files, so I won't commit too much formatting changes into this PR. I'll sneak those in with a commit afterwards.