Consistently use slurm_mutex_ macros rather than direct pthread_mutex functions.

src/api/allocate_msg.c

src/api/pmi.c

src/api/pmi_server.c

src/api/step_io.c

src/api/step_launch.c

src/plugins/burst_buffer/cray/burst_buffer_cray.c

src/plugins/burst_buffer/generic/burst_buffer_generic.c

src/plugins/checkpoint/poe/checkpoint_poe.c

src/plugins/jobcomp/script/jobcomp_script.c

src/plugins/mpi/mpich1_p4/mpich1_p4.c

src/plugins/mpi/mvapich/mvapich.c

src/plugins/mpi/pmix/pmixp_coll.c

src/plugins/mpi/pmix/pmixp_state.c

src/plugins/power/cray/power_cray.c

src/plugins/sched/backfill/backfill.c

src/plugins/sched/backfill/backfill_wrapper.c

src/plugins/sched/builtin/builtin.c

src/plugins/sched/builtin/builtin_wrapper.c

src/plugins/sched/wiki/msg.c

src/plugins/sched/wiki2/event.c