unify static and dynamic linked implementations of thread-local storage

unify static and dynamic linked implementations of thread-local storage

this both allows removal of some of the main remaining uses of the
SHARED macro and clears one obstacle to static-linked dlopen support,
which may be added at some point in the future.

specialized single-TLS-module versions of __copy_tls and __reset_tls
are removed and replaced with code adapted from their dynamic-linked
versions, capable of operating on a whole chain of TLS modules, and
use of the dynamic linker's DSO chain (which contains large struct dso
objects) by these functions is replaced with a new chain of struct
tls_module objects containing only the information needed for
implementing TLS. this may also yield some performance benefit
initializing TLS for a new thread when a large number of modules
without TLS have been loaded, since since there is no need to walk
structures for modules without TLS.