ldso: avoid spurious & possible erroneous work for libs with no deps

ldso: avoid spurious & possible erroneous work for libs with no deps

a null pointer for a library's deps list was ambiguous: it could
indicate either no dependencies or that the dependency list had not
yet been populated. inability to distinguish could lead to spurious
work when dlopen is called multiple times on a library with no deps,
and due to related bugs, could actually cause other libraries to
falsely appear as dependencies, translating into false positives for
dlsym.

avoid the problem by always initializing the deps pointer, pointing to
an empty list if there are no deps. rather than wasting memory and
introducing another failure path by allocating an empty list per
library, simply share a global dummy list.

further fixes will be needed for related bugs, and much of this code
may end up being replaced.