Added a flag field to the opcode macros and an instruction flag I_PC_AT_NEXT

to use for opcodes that update Pike_fp->pc to the address of the next
opcode.

Rev: src/interpret.c:1.258
Rev: src/interpret_functions.h:1.101
Rev: src/interpreter.h:1.75
Rev: src/lex.c:1.103
Rev: src/lex.h:1.21
Rev: src/opcodes.h:1.27
Rev: src/strip_opcodes:1.3