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 did ARG_WITH so that it is grouped with the "Optional packages" ./configure output, like:

    Optional Packages:
      --with-PACKAGE[=ARG]    use PACKAGE [ARG=yes]
      --without-PACKAGE       do not use PACKAGE (same as --with-PACKAGE=no)
      --with-valgrind         Enable valgrind annotations [default=no]
      --with-pic              try to use only PIC/non-PIC objects [default=use
                              both]
      --with-gnu-ld           assume the C compiler uses GNU ld [default=no]
      --with-dlopen           dl-loadable provider support [default=yes]
      --with-extdir           specify directory with loadable providers to scan
                              [default=$pkglibdir]

This seems to go with the these vs the providers.

As for aborting if no dl support is selected and providers are built =dl ? I thought of this but figured the user should be allowed this flexibility if they want, I think the chances of shooting yourself in the foot are small and/or well-deserved.