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 think it would be ok/good to put in a call to stat(2) and check for a) the path, b) that it's a directory, and c) that it's readable.  I don't think that people should be slaved to non-DSO builds just if they're concerned about fat-fingering an environment variable name.

Put simply: silent errors are bad.