btl: Use C99 sub-object naming when initializing BTL components

Two things to note:

 - This change will allow us to expand the BTL interface without
   having to worry about modifying BTLs that will not support the new
   interfaces. More on this will come later this year as part of the
   1.9 series.

 - C99 guarantees that uninitialed members of structs declared outside
   of functions (DATA binary section) will be initialized with
   0's. This allows us to drop stuff like .btl_flags = 0, or .btl_get
   = NULL.
Change this filename back (i.e., revert r31375).

It's a singular filename because there's only 1 interface in the
file.  Also, r31375 missed updating the name in a few places, and
broke the build for compilers that supported the mpi_f08 interface.