Add a select_plugin_type to give a enum for the different select plugins.

slurm/slurm.h.in

src/plugins/select/alps/select_alps.c

src/plugins/select/bluegene/select_bluegene.c

src/plugins/select/cons_res/select_cons_res.c

src/plugins/select/cray/select_cray.c

src/plugins/select/linear/select_linear.c

src/plugins/select/serial/select_serial.c