plugin-loader: Optionally use RTLD_NOW with dlopen()

plugin-loader: Optionally use RTLD_NOW with dlopen()

This can be useful when writing custom plugins as typos or missing
linker flags that result in unresolved symbols in the shared object
could otherwise cause late crashes.  In particular, if such a symbol
is used in a code path that is rarely executed.  During development
and testing using RTLD_NOW instead of RTLD_LAZY will prevent the
plugin from getting loaded and makes the error visible immediately.