gdb: Reduce the recent changes to only the necessary bits in amd64dfly-nat.c.

This makes maintenance of base and dports gdb easier.

Suggested-by: marino
Discussed-with: tuxillo

gdb - Fix indentantion in DragonFly specific files.

- No functional changes

gdb: Add modifications required for version 7.6.1

THe majority of these modifications (the 4 "dfly" files and osabi.c)
are needed just to get the stock gdb to build. The modification to
inferior.c

gdb: Add modifications required for version 7.6.1

THe majority of these modifications (the 4 "dfly" files and osabi.c)
are needed just to get the stock gdb to build. The modification to
inferior.c was reintroduced so kgdb can handle the 0 PID swapper
process, but the modication is much less than with version 7.4.1.
The change to the ptid_t structure (ptid.h) was necessary because
kgdb uses the thread address as the thread id, so the type has to
cover the entire address space.

The DragonFly README files were also updated.

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gdb: add our changes