* make mission log dynamic
Change the mission log from a static array to a dynamic vector. This removes any chance of the mission log running into a limit (as regularly happened with the last mission of Shepherds before the limit bump) and also prevents strange mission bugs caused by culling obsolete entries.
This also unifies the API a bit by renaming the public `message_log_` function prefixes to `mission_log_`, removing the redundant `Num_log_lines` variable, and removing `Log_scrollback_vec` from the header.
Implements #5878.
* feedback
* refactor to reference rather than pointer
Many of the recent scripting API enhancements are missing validity functions and validity checks, so this adds them. There are also some documentation fixes and a >= vs. > bugfix.