Cleaned up and wrote some helpful notes about the lvalue handling.

Rev: src/builtin.cmod:1.120
Rev: src/code/ia32.c:1.24
Rev: src/interpret.c:1.291
Rev: src/interpret_functions.h:1.138
Rev: src/object.c:1.225
Rev: src/pike_types.c:1.207
Rev: src/svalue.c:1.159
Rev: src/svalue.h:1.116