Compiler bug workaround for execvp call.

Specifically,
	const char *args[18];
	...
	execvp("mterm",(void *)&args[0]);
draws a warning from 1.4T's compiler about the cast discarding const
from the pointer target type.  This warning is incorrect (the
pointed-to type is non-const - presumably the compiler is confusing
const char ** with char * const * or const char * const * - but, since
execvp's second arg is char * const * on all OSes I care about, just
casting to const char * is a tolerable workaround.