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.
Yes that is wrong. What I wanted was a way to silently ignore dir not found, but not any other errors (permission, I/O, etc). This could be used to disable dl-loading at runtime by setting FI_EXTDIR=/dev/null (there have been some performance concerns about scandir-ing over network mounted filesystem).

Thinking about this more though: I think the better option would for people concerned with this would be to build with dl-open disabled.