Document that DynaLoader::dl_unload_file() is not called at exit by default.

DynaLoader::dl_unload_file() is only called automatically to unload all
loaded shared objects if the perl interpreter was built with the C macro
DL_UNLOAD_ALL_AT_EXIT defined. The documentation has been incorrectly
stating that it is always called since dl_unload_file() was added in v5.6.0

When dl_unload_file() was first added by commit abb9e9dca5a5f121 (March 1st,
2000), it was always called at exit, if available. It was made conditional on
DL_UNLOAD_ALL_AT_EXIT being defined by commit 23d2500b2b45b1be (March 20th,
2000), but the documentation was never updated to reflect this. v5.6.0 was
release on March 22nd, 2000.
Whitespace changes to DynaLoader.t, as part of adding a TODO: block.
Skip tests for dl_unload_file() when built with -DPERL_GLOBAL_STRUCT_PRIVATE

Building with -DPERL_GLOBAL_STRUCT_PRIVATE uses alternative implementations
of Perl_my_cxt_init() and Perl_my_cxt_index() which store the pointer to the
extension name. For a dynamically loaded shared object this is a pointer to
memory which will be released if that shared object is unloaded, meaning that
any *other* module which calls Perl_my_cxt_index() will likely attempt a read
of unmapped memory.