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.
Also, if --with-dl=yes, then there should be a cross-check to ensure that no provider tries to build itself as a DL (if they do, we should AC_MSG_ERROR/abort).