headers: Beautify sigframe.h
headers: Beautify sigframe.h

- include "protobuf/core.pb-c.h" for CoreEntry definition
- gather structure predeclaration on top
- move general comment on top as well
- add extern for construct_sigframe helper

Signed-off-by: Cyrill Gorcunov <7a1ea01eee6961eb1e372e3508c2670446d086f4@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

wtf42/criu,rentzsch/criu,kawamuray/criu,gonkulator/criu,gonkulator/criu,AuthenticEshkinKot/criu,eabatalov/criu,gablg1/criu,kawamuray/criu,svloyso/criu,marcosnils/criu,LK4D4/criu,biddyweb/criu,KKoukiou/criu-remote,fbocharov/criu,gablg1/criu,ldu4/criu,AuthenticEshkinKot/criu,svloyso/criu,ldu4/criu,marcosnils/criu,svloyso/criu,fbocharov/criu,rentzsch/criu,biddyweb/criu,tych0/criu,KKoukiou/criu-remote,kawamuray/criu,marcosnils/criu,LK4D4/criu,AuthenticEshkinKot/criu,kawamuray/criu,wtf42/criu,svloyso/criu,tych0/criu,rentzsch/criu,marcosnils/criu,AuthenticEshkinKot/criu,eabatalov/criu,marcosnils/criu,gonkulator/criu,gablg1/criu,eabatalov/criu,LK4D4/criu,sdgdsffdsfff/criu,gablg1/criu,wtf42/criu,biddyweb/criu,ldu4/criu,biddyweb/criu,gonkulator/criu,tych0/criu,rentzsch/criu,KKoukiou/criu-remote,AuthenticEshkinKot/criu,wtf42/criu,biddyweb/criu,ldu4/criu,LK4D4/criu,efiop/criu,eabatalov/criu,kawamuray/criu,ldu4/criu,efiop/criu,sdgdsffdsfff/criu,KKoukiou/criu-remote,sdgdsffdsfff/criu,marcosnils/criu,gonkulator/criu,KKoukiou/criu-remote,sdgdsffdsfff/criu,wtf42/criu,tych0/criu,svloyso/criu,fbocharov/criu,fbocharov/criu,rentzsch/criu,sdgdsffdsfff/criu,gonkulator/criu,ldu4/criu,sdgdsffdsfff/criu,rentzsch/criu,wtf42/criu,efiop/criu,efiop/criu,AuthenticEshkinKot/criu,efiop/criu,KKoukiou/criu-remote,fbocharov/criu,fbocharov/criu,efiop/criu,gablg1/criu,svloyso/criu,biddyweb/criu,eabatalov/criu,LK4D4/criu,tych0/criu,LK4D4/criu,gablg1/criu,tych0/criu,kawamuray/criu,eabatalov/criu