Previously, it only pretended to rewrite the configuration file, so it
set or cleared OPT_MUST_SAVE but never changed or output any options.
Now, it actually sets the options when ELinks is loading the
configuration file. Also, when ELinks is rewriting the configuration
file, it now compares the values in the included file to the current
values of the options, and sets or clears OPT_MUST_SAVE accordingly.
So, if elinks.conf contains a "set" command for an alias and ELinks
updates that, it now knows it doesn't have to append another "set"
command for the underlying option.
Also, replace OPT_WATERMARK with OPT_MUST_SAVE, which has the opposite
meaning.
Watermarking of aliases does not yet work correctly in this version.
Neither does the "include" command.
Previously, they were reset by smart_config_string(), which was not
called if the value of the option was saved by rewriting an existing
command in elinks.conf. Also, it is better to reset the flags only
after the file operations have actually succeeded.
Previously, ELinks set the OPT_WATERMARK flag in all deleted options
when config.saving_style was 2, thus mostly preventing them from being
saved. This had the unfortunate consequence that if you started with
no elinks.conf, set config.saving_style = 2, deleted some built-in
option (e.g. a URL rewriting rule), saved the settings, and restarted
ELinks, then the built-in option would reappear.
