rename global "objname" variable to "childobj"

There are two distinct usages of "objname" as a variable name:

- the parent object being patched (e.g. vmlinux)
- the child object being analyzed (e.g. meminfo.o)

The name of the global objname variable conflicts with several
functions' usage of a local objname variable, resulting in some error
messages of e.g., "ERROR: vmlinux:" instead of "ERROR: meminfo.o:".

Rename the global objname variable to childobj.