Minor changes to aid use of dmalloc library

PCB supports the use of the dmalloc library as a configuration option. Thsi patch makes the PCB source code more "friendly" to libdmalloc in the following ways:
* Some source files that allocate memory from the heap but fail to include <dmalloc.h> now do so
* The mymem.c low level memory routines are replaced with corresponding libdmalloc calls (if libdmalloc is available). This is helpful as these routines otherwise obscure the __FILE__ and __LINE__ information of lines that use the heap.