Fix UNIX file open
Fix UNIX file open

In #10973 I refactored FileAccessUnix::_open() but I accidentally made
it impossible to create new files.

This fixes that and fixes #10984

Paulloz/godot,godotengine/godot,okamstudio/godot,MarianoGnu/godot,vkbsb/godot,okamstudio/godot,RandomShaper/godot,FateAce/godot,honix/godot,zicklag/godot,Paulloz/godot,akien-mga/godot,ex/godot,n-pigeon/godot,guilhermefelipecgs/godot,BastiaanOlij/godot,sanikoyes/godot,godotengine/godot,jejung/godot,vnen/godot,zicklag/godot,ageazrael/godot,DmitriySalnikov/godot,n-pigeon/godot,ex/godot,mcanders/godot,firefly2442/godot,BastiaanOlij/godot,Valentactive/godot,MarianoGnu/godot,Valentactive/godot,ZuBsPaCe/godot,Brickcaster/godot,DmitriySalnikov/godot,Zylann/godot,josempans/godot,RandomShaper/godot,guilhermefelipecgs/godot,sanikoyes/godot,godotengine/godot,zicklag/godot,honix/godot,josempans/godot,Valentactive/godot,pkowal1982/godot,firefly2442/godot,Valentactive/godot,Faless/godot,pkowal1982/godot,NateWardawg/godot,ZuBsPaCe/godot,ex/godot,honix/godot,n-pigeon/godot,ZuBsPaCe/godot,akien-mga/godot,MarianoGnu/godot,honix/godot,NateWardawg/godot,vnen/godot,josempans/godot,okamstudio/godot,firefly2442/godot,NateWardawg/godot,Paulloz/godot,BastiaanOlij/godot,okamstudio/godot,Shockblast/godot,zicklag/godot,honix/godot,FateAce/godot,groud/godot,sanikoyes/godot,ZuBsPaCe/godot,okamstudio/godot,MarianoGnu/godot,Paulloz/godot,akien-mga/godot,Zylann/godot,pkowal1982/godot,vnen/godot,godotengine/godot,Valentactive/godot,Faless/godot,BastiaanOlij/godot,RandomShaper/godot,mcanders/godot,NateWardawg/godot,Paulloz/godot,DmitriySalnikov/godot,zicklag/godot,mcanders/godot,FateAce/godot,NateWardawg/godot,guilhermefelipecgs/godot,jejung/godot,groud/godot,Brickcaster/godot,sanikoyes/godot,groud/godot,ageazrael/godot,Paulloz/godot,vnen/godot,Zylann/godot,firefly2442/godot,Zylann/godot,groud/godot,godotengine/godot,Faless/godot,Valentactive/godot,BastiaanOlij/godot,MarianoGnu/godot,Valentactive/godot,jejung/godot,Brickcaster/godot,mcanders/godot,NateWardawg/godot,Shockblast/godot,guilhermefelipecgs/godot,n-pigeon/godot,vnen/godot,RandomShaper/godot,ageazrael/godot,guilhermefelipecgs/godot,Zylann/godot,Shockblast/godot,n-pigeon/godot,DmitriySalnikov/godot,FateAce/godot,sanikoyes/godot,ageazrael/godot,vkbsb/godot,firefly2442/godot,Shockblast/godot,FateAce/godot,Brickcaster/godot,NateWardawg/godot,ageazrael/godot,ex/godot,vkbsb/godot,MarianoGnu/godot,honix/godot,Paulloz/godot,vnen/godot,pkowal1982/godot,Shockblast/godot,godotengine/godot,Brickcaster/godot,jejung/godot,vkbsb/godot,vnen/godot,vkbsb/godot,groud/godot,Shockblast/godot,BastiaanOlij/godot,Faless/godot,akien-mga/godot,ex/godot,godotengine/godot,Brickcaster/godot,akien-mga/godot,MarianoGnu/godot,pkowal1982/godot,Faless/godot,vnen/godot,BastiaanOlij/godot,Zylann/godot,akien-mga/godot,n-pigeon/godot,BastiaanOlij/godot,josempans/godot,mcanders/godot,ageazrael/godot,firefly2442/godot,ex/godot,vkbsb/godot,ageazrael/godot,josempans/godot,pkowal1982/godot,RandomShaper/godot,sanikoyes/godot,pkowal1982/godot,okamstudio/godot,NateWardawg/godot,ZuBsPaCe/godot,akien-mga/godot,ex/godot,NateWardawg/godot,DmitriySalnikov/godot,firefly2442/godot,josempans/godot,Faless/godot,jejung/godot,jejung/godot,okamstudio/godot,zicklag/godot,firefly2442/godot,Zylann/godot,Shockblast/godot,ZuBsPaCe/godot,Brickcaster/godot,Valentactive/godot,guilhermefelipecgs/godot,vkbsb/godot,DmitriySalnikov/godot,sanikoyes/godot,ex/godot,mcanders/godot,n-pigeon/godot,ZuBsPaCe/godot,godotengine/godot,Zylann/godot,okamstudio/godot,josempans/godot,groud/godot,vkbsb/godot,RandomShaper/godot,guilhermefelipecgs/godot,josempans/godot,okamstudio/godot,akien-mga/godot,RandomShaper/godot,Faless/godot,FateAce/godot,pkowal1982/godot,okamstudio/godot,DmitriySalnikov/godot,ZuBsPaCe/godot,sanikoyes/godot,guilhermefelipecgs/godot,MarianoGnu/godot,Faless/godot,Shockblast/godot