Do not leak %rdx value in the previous image to the new image after execve(2). Note that ia32 binaries already handle this properly, since ia32_setregs() resets td_retval[1], but not exec_setregs().
Do not leak %rdx value in the previous image to the new image after
execve(2). Note that ia32 binaries already handle this properly,
since ia32_setregs() resets td_retval[1], but not exec_setregs().

We still do not conform to the amd64 ABI specification, since %rsp
on the image startup is not aligned to 16 bytes.

PR:	amd64/124134
Discussed with:	Petr Salinger <Petr.Salinger seznam cz>
	(who convinced me that there is indeed several bugs)
MFC after:	1 week

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase