historysize

[Petr M. <mi...@ph...>]

Patch "History file for gnuplot readline" has been just committed. So both 
readlines can use it.

I'm bit surprised what "unset historysize" is doing. It sets historysize to 
unlimited, not suppressing writing the history file.

I propose the following changes:
1. The code for history file is always compiled in (thus no need to
    --enable-history-file nor to modify dozen of files in config/).
2. By default, the history file is not read. It can be switched on in
    .gnuplot by "set historysize"
3. Switching off writing/reading history file: "unset historysize"
4. "set historysize -1" would set historysize to unlimited.

Ideas?

---
PM