Update config parsing methods to ensure that any built-in functions that allocate memory for linked list data structures don't leak/orphan memory with subsequent calls

[morgant]

As discovered in Issue #45, SetStartFunction() in mlvwm/config.c contains a bug which can orphan linked list objects, leaking memory, with subsequent parsing of InitFunction/RestartFunction built-in function blocks in config files. This is because it references a pointer stored in the Scr variable's data structure, then allocates memory for a new object into that referenced variable in the first iteration of the parsing loop, effectively starting a new linked list instead of appending to the existing one, if one exists. Unfortunately, while it intentionally replaces the old linked list of parsed configuration data/settings upon subsequent executions, it doesn't actually free any prior linked list objects if they already exist.

Since this parsing into linked lists of data/settings is a common pattern in the mlvwm implementation, I suspect this bug occurs in a number of the other configuration parsing methods in mlvwm/config.c. Upon completing issue #45, let's look for and fix any similar memory leak bugs en masse.