localboot: Clear register image before using

Using a register image on the stack without initializing it is not a
good idea.

Reported-by: Erwan Velu <erwanaliasr1@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Merge remote-tracking branch 'origin/syslinux-5.xx'
com32sys_t inreg shall be zeroified prior intcall

As per commit f775e740a3a817a4ff5ba26bea99dbfd735456b3, inreg parameters
of intcall() shall be zeroified.

Having unclean inreg could trigger bad behaviors on some hosts.

This patch is about adding memset() calls prior any intcall() :
- some intcall didn't had any memset at all
- some successive intcall() calls didn't memset inreg in between calls
mem: Calling int15 for 0x8800

On some code, the intcall 0x15 was missing when playing with 0x8800
making code useless.

That's not a big deal since e820 or e801 shall be provided by host but
anyway, let's fix it ;)