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.
I just looked at the GNU Autoconf docs -- looks like they have a specific use-case defined for --with:

> 15.2 Working With External Software
> Some packages require, or can optionally use, other software packages that are already installed. The user can give configure command line options to specify which such external software to use. The options have one of these forms:
> --with-package[=arg]
> --without-package
> ...
> For each external software package that may be used, ‘configure.ac’ should call AC_ ARG_WITH to detect whether the configure user asked to use it. Whether each package is used or not by default, and which arguments are valid, is up to you.

And for --enable:

> 15.3 Choosing Package Options
> If a software package has optional compile-time features, the user can give configure command line options to specify whether to compile them. The options have one of these forms:
> --enable-feature[=arg]
> --disable-feature

From that perspective, it sounds like we should use --enable for dl support (and for all the providers, which we already do).

I don't think we honor this convention in OMPI... oops... :-(