gdb: Add modifications required for version 7.6.1

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.