sysdump: Include stdout/stderr of cilium sysdump in sysdump

[pchaigno]

When we ask users to retrieve a sysdump, we are sometimes missing information in the sysdump (such as bugtool reports) because errors happened. If we also collected the stdout and stderr of the cilium sysdump command in the sysdump itself, we would know exactly what errors were hit and could improve cilium sysdump or ask to retrieve a new sysdump.

[borkmann]

@tklauser could you or @sayboras take a look? Thx

[tklauser]

Should be fixed by #728