Mark static modules with a different type, and check that type at unload.
Only call unload low level implementation if type is dynamic.

Add flags to indicate symbol visibility for OS_ModuleLoad().

By default (flags=0) symbols will have global visibility,
which should be the same as existing behavior.

This restores the original behavior when using OS_SymbolLookup on POSIX,
so it doesn't return OS_ERR_NOT_IMPLEMENTED.

This is required for transitional purposes.  Will submit a separate bug
about use platform-dependent logic here.

Fix #637, fix OS_ModuleUnload for static modules

Fix #641, add OS_ModuleLoad flags

Add doxygen comment for flags parameter on OS_ModuleLoad

Also correct the capitalization of parameters to the
OS_ModuleSymbolLookup which caused them to be
reported as undocumented.