memory debugging examples
- first install libmem from directory 04.libmem
- there are two programs (testmem fails to free, testmem2 frees twice)
- run them without libmem, first one "succeeds" (but leaks memory)
  second one dies with a very nasty gnu C library "double free
  detected" result with pages of horrid output.
- add libmem:
  - add -lmem to LDLIBS in Makefile
  - add (uncomment) #include <mem.h> lines in *.c
  - make clean all
- rerun - libmem detects both problems cleanly.
