termkey_lookup_keyname() looks up the symbolic key value represented by the given string name. This is a
case-sensitive comparison. The symbolic value is written to the variable addressed by sym. This function
is a more general form of termkey_keyname2sym(3) because it can recognise a symbolic key name within a
longer string, returning a pointer to the remainder of the input after the key name.
Because the key names are stored in an array indexed by the symbol number, this function has to perform a
linear search of the names. Use of this function should be restricted to converting key names into
symbolic values during a program's initialisation, so that efficient comparisons can be done while it is
running.