Add logic to launch slurmstepd under valgrind

The #define of SLURMSTEPD_MEMCHECK in src/slurmd/common/slurmstepd_init.h
must be changed to enable memcheck or valgrind. Also change a #if in
src/slurmd/slurmd/req.c near where you find the "valgrind" references.
Bug 2334 diagnostics