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.
Hmm.  I think this might be wrong for 2 reasons:

1. According to scandir(3), -1 is returned on any error, not just for dir-not-found.
1. It shouldn't be a silent error if the user typo's the dirname in $FI_EXTDIR.  Perhaps we should stat(2) the dir to ensure it exists, is readable, and is a directory first.  And if it's not, then print a warning (or error? shrug).