fix appendfile problem on macos.
when i open file with mode 'a+' on linux, seek position is 0.
on macos seek position is end of file instead. not documented.
default = 'changelog'
header = header-rss.tmpl
changelog = changelog-rss.tmpl
changelogentry = changelogentry-rss.tmpl
filelog = filelog-rss.tmpl
filelogentry = filelogentry-rss.tmpl
tags = tags-rss.tmpl
tagentry = tagentry-rss.tmpl