On some implementation of `dladdr`, the `dli_sname` and `dli_fname`
can be null even if the `dladdr` itself succeed.

FreeBSD implemented DTrace fairly standardly in compare to Illumos.

1. build.rs
   This patch enable FreeBSD to be treated as a "standard" implementation
   with no linker supported in the usdt build.rs.

2. The values to issue ioctl to dtrace helper are different.
   2.1. The cmd value on FreeBSD is derived the C macro
        `_IOWR('z', 3, dot_helper_t)`;  which is unfolded and the value
        is inlined in this patch.
   2.2 The struct dof_helper is also different, in FreeBSD there are
       two extra fields: `dofhp_pid` and `dofhp_gen`.

3. Linker Issue
   Like Illumos, without `FORCE_LOAD` line the linker will omit the symbols
   (even with "-C link-dead-code"); therefore the line also needed to activate
   on FreeBSD.

   There are however still issues in linker that make the FreeBSD support
   preliminary, on FreeBSD the crate consumer need to build with
   "-C link-dead-code", or otherwise the probes will thrown away by the linker.

Provide a macro version of register_probes!() enables a explicit reference to
the set_dtrace_probes section from the crate consumer, and prevent lld from gc
the section. This trick along with the unstable "used(linker)" feature enable
use on FreeBSD target with nightly toolchain without explicit "link-dead-code"

Since the "used_with_arg" is available only on nightly, a patch is applied on
usdt-impl/build.rs such that standard backend is enabled only for FreeBSD
targets that are either on nightly or have link-dead-code explicitly passed to
the linker.

Turns out we do not need all the hacks to get FreeBSD linker working.
GNU and llvm both support the "R" flag for sections, which prevent
them from gc.

This make the ugly register_probes! hack obsolete so it can be removed.
No one deserve to have it in their codebase.

https://lists.freebsd.org/archives/freebsd-hackers/2022-June/001191.html

FreeBSD dladdr(3M) examine dynamic symbol table only hence never give use the mangled symbol.
Instead, we use `backtrace_symbols_fmt` to give us the symbol.

Handle backtrace_symbols_fmt and ffi a bit more carefully