shmem support routines moved to shmem_utils

Some cleanup done on the shmem implementation, mainly moving support routines
from smalloc, domdec and domdec_network to shmem_utils.c. Now the header shmem_utils.h
is the only one required when using shmem within GROMACs.
Tested with 4p.
Apparently there are problems with 32p and some regtests, needs further testing.