Add ecmascript_interpreter.backend_nesting, increment it when
beginning to evaluate an expression, and decrement it when evaluation
finishes. Then assert that it is zero in ecmascript_put_interpreter.
This detects bug 957 and similar ones before they corrupt memory.
straconcat reads the args with va_arg(ap, const unsigned char *),
and the NULL macro may have the wrong type (e.g. int).
Many places pass string literals of type char * to straconcat. This
is in principle also a violation, but I'm ignoring it for now because
if it becomes a problem with some C implementation, then so will the
use of unsigned char * with printf "%s", which is so widespread in
ELinks that I'm not going to try fixing it now.
Otherwise if the page installs multiple timers the old one would live
on unreferenced and possibly (likely) trigger after the document's death
and everything would go to hell.
Replace the ECMAScript module initialisation and de-initialisation
routines that wrapped the SMJS and SEE module initialisation and
de-initialisation routines by having the module system call the SMJS
and SEE routines its own darned self.
