Rename zero_realloc option "strict" to "alloc".

With realloc(ptr, 0) being UB per C23, the option name "strict" makes less sense
now.  Rename to "alloc" which describes the behavior.

test/unit/zero_realloc_alloc.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+export MALLOC_CONF="zero_realloc:alloc"