Limit vim modeline salvage to header section.

There might be legitimate usages of vim modelines in the middle of
a spec file (e.g. in inline config file sections), so best not mess
with that all over the file. Case in point:

https://build.opensuse.org/package/view_file/Apache:Modules/apache2-mod_fastcgi/apache2-mod_fastcgi.spec?expand=1