Add new function to node selection plugin: select_p_part_init() so

doc/html/selectplugins.html

src/plugins/select/bluegene/select_bluegene.c

src/plugins/select/linear/select_linear.c

src/slurmctld/partition_mgr.c

src/slurmctld/read_config.c

src/slurmctld/select_plugin.c

src/slurmctld/select_plugin.h