Following 2046b45aa0, this change introduces better control of memory
allocation flags for escaped values. Notably:
- The e->is_args flag is now explicitly reset on rewrite start.
If the flag was set prior to rewrite start, then buffer overflow
could happen before 2046b45aa0.
- The le->is_args flag value is now copied from e->is_args when
calculating complex value length for "if" and "set" directives.
If e->is_args was set, but le->is_args was not, then buffer overflow
could happen before 2046b45aa0.
