Make dlopen() use cwd.

Without this fix the program will get a segmentation fault, as
dlopen() will return NULL, and dlerror() diagnoses:

    Failed to load fn.so: fn.so: cannot open shared object file: No such file or
    directory

man dlopen says:

    If filename contains a slash ("/"), then it is interpreted as a (relative or
    absolute) pathname (**what we want**). Otherwise, the dynamic linker
    searches for the object as follows… (*lists several options that don't for
    the cwd unless you set up env vars*)