delta_list_rm() bugfix

prev pointers were not being updated in delta_list_rm().  Should be OK
now.
Clean up time.c

Add documentation, fix a few potential sources of bugs.
Improve makefile

Factor out more common dependencies
Improve sleep_test()
Implement (v)s(n)printf()

Only strings, chars, (base-10) integers and literal '%'s for the time
being.  Nothing fancy (no precision specifiers, etc.).