Add strldup() and safestrldup(), reducing ridiculous amount of code in

Add strldup() and safestrldup(), reducing ridiculous amount of code in
m_pass and m_topic.c when duplicating strings with a length limit.
+/* strldup(str,max) copies a string and ensures the new buffer
+ * is at most 'max' size, including nul byte. The syntax is pretty
+ * much identical to strlcpy() except that the buffer is newly
+ * allocated.
+ * If you wonder why not use strndup() instead?
+ * I feel that mixing code with strlcpy() and strndup() would be
+ * rather confusing since strlcpy() assumes buffer size including
+ * the nul byte and strndup() assumes without the nul byte and
+ * will write one character extra. Hence this strldup(). -- Syzop
+ */