outobj: make a group cumulative

[komh]

On any other OMF assemblers such as MASM, TASM and ALP, a group is cumulative.

[hpax]

To be frank, I don't even really understand what "groups are cumulative" mean. If you could put something in changes.src (and, if necessary, in outfmt.src) that would be really helpful.

[ecm-pushbx]

@hpax I think they mean that multiple group directives naming the same group can be used, and all section names listed in any of them are accumulated into the same group. I came across this during some work porting source texts to NASM. NASM currently rejects multiple group directives naming the same group.

[komh]

For example,

group DGROUP .data
group DGROUP .bss
group DGROUP .data

should put .data, .bss in DGROUP without any problems. Currently, NASM does not allow this. I'll add this explanation to changes.src and outfmt.src.

[komh]

I've documented for cumulative GROUP directive.

[hpax]

Merged.