Rewrite: harden escape flags control

Rewrite: harden escape flags control

Following 2046b45aa0c6, 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 2046b45aa0c6.

- 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 2046b45aa0c6.