Fix startup crash when running outside of gdb on BSD systems

On BSD systems, ptrace(PT_DETACH) uses a third argument for
resume address, with the magic value (void *)1 to resume where
process stopped. Specifying NULL there leads to a crash because
process resumes at address 0.

We introduce an OS-dependent _PTRACE_DETACH macro to specify
third argument as NULL on Linux and (void *)1 on other systems.
Always using (void *)1 could be another solution, since basic
tests suggests passing (void *)1 as third argument on Linux
does not cause harm.

From upstream https://github.com/FreeRADIUS/freeradius-server/commit/1f8cd27b78c011826bc0744debb4ed60b3f0584a

(manu)