Upload announcements broken with \

[wspr]

Description

From Petra regarding my last upload:

 Seems to have worked fine, in principle, except for `\verbvisiblespace`,
 which was rendered as `^Kerbvisiblespace` in the upload notification
 email for maintainers  and as  `
                                 erbvisiblespace`
 in a metadata file on the server.

[davidcarlisle]

I think l3build upload should double up \ if reading from a file (or perhaps always?) If just doing it for files then:

  -- Get data from command line if appropriate
  if options["file"] then
    local f = open(options["file"],"r")
    uploadconfig.announcement = assert(f:read('*a'))
    uploadconfig.announcement=string.gsub(uploadconfig.announcement,'\\','\\\\') -- this line added
    close(f)
  end

Curl manual says

If the parameter contains whitespace (or starts with : or =), the parameter must be enclosed within quotes. Within double quotes, the following escape sequences are available: \\, \", \t, \n, \r and \v. A backslash preceding any other letter is ignored. If the first column of a config line is a '#' character, the rest of the line will be treated as a comment. Only write one option per physical line in the config file.