SWAT-352: when loading lleventhost, call apr_dso_error() function.

In addition to its usual apr_sterror() function, APR defines a special
function specifically for errors relating to the apr_dso_*() functions.
Introduce ll_apr_warn_status() and ll_apr_assert_status() overloads accepting
apr_dso_handle_t* to call apr_dso_error() as well as apr_strerror() and log
its output. Use new ll_apr_warn_status() in LLAppViewer::loadEventHostModule()
for apr_dso_load() and apr_dso_sym() calls. Instead of shorthand
ll_apr_assert_status(), use with llassert_always() so check is still performed
even in Release build.
Add more lleventhost-related debugging output, e.g. full pathname of the DLL.
On Mac and Linux, call 'file' command to report nature of the DLL too.