added exe-only options to win32pe generation

"msfencode -t exe" or "-t exe-small" are now detected by AV regardless of the template.
This is a patch "-t exe-only" that insert payload at the template entry point adding flag writable on the section Characteristics.
So only payload can be detected by AV.

RWX sections could increase the detection but it can't be the only criterion of detection.

- [ ] Choose a PE (not native windows binary nor already AVs detected binary obviously)
- [ ] Change .text section Characteristics : add W flag (0x800000000) LordPE can do it...
- [ ] Scan the PE with every AV you want
- [ ] Beat me (...or not I hope)