dtcreate: Fix some memory management issues

dtcreate: Fix some memory management issues

ProcessExecString thought it was returning an array of size 3; however
in C arrays are second-class and there is no direct way to return an
array like this; GCC warning triggered because it was actually
just returning a pointer to local storage. Fixed using malloc.

Also fix some obviously wrong usages of sizeof, although they were
relatively harmless. A little other warning quieting using 0 instead of
NULL.