Clean up dl-open features:

Add --with-dlopen=[yes,no] option to compile without loadable provider support.
Selecting no also disables the check for -ldl.

Add --with-extdir option to specify custom loadable provider dir.
Default is $libdir/libfabric.

Rename FI_EXTPATH to FI_EXTDIR for consistency with variable names and configure options.

Silently ignore extdir not found when loading instead of printing error to stderr.

Remove unused includes in fabric.c

Re-include common.c and enosys.c in dl-loadable providers, these are needed.
You want to override it at *configure* time, not *compile time*, right?

I hear what you're saying, but I guess the real question is: is there a use case for *not* wanting to install the DSOs in $(libdir)/libfabric?

I ask because I think the reason there's no --pkglibdir option is because it's *supposed* to be chained to libdir.