Move syscall_args into struct thread.

[kib]

Summary

This allows to remove td_dbg_sc_code and td_dbg_sc_nargs.
Also, it allows to optionally print the syscall number in nosys().
Also, add PT_GET_SC_ARGS argument fetch operation for ptrace(2).

[kib]

Right now this is only compiled on i386/amd64. PT_GET_SC_ARGS is not tested and not documented.

[kib]

Handle compat32: simply copy out low 32bits from each arg. This should be, in fact, a correct truncation.

[kib]

Adapt !x86 arches.

[kib]

Document PT_GET_SC_ARGS.

[jhb]

Looks good to me. For full truss cleanup I'll also eventually want a PT_GET_SC_RVAL that returns td_retval[] and the translated error value from the system call (though I could also accept the "raw" error and do the translation in userland via the tables in libsysdecode). That one is easy enough I can add when adopting truss to use this.

[kib]

Update patch to the current state, including fixes to PT_GET_SC_ARGS (it passes tests) and to scescx.c test itself.