This prevents loading a spurious tag for the format specifier line, as
well as fixing loading of CTags format with a format specifier line.

Before this change, the file pointer was rewound after reading a format
specifier line; but this had various unwanted side effects depending on
the recognized format:

* For TagManager and Pipe formats, it led to loading a tag named after
  the format specifier (e.g. a literal "# format=tagmanager").  This
  was fairly harmless and only introduced a spurious tag seldom even
  used because "#" isn't usually considered for looking up completions.
* For CTags format, having an explicit specifier led to failure to load
  the file in most cases because the specifier line would be parsed but
  doesn't usually follow the format's requirements, leading to early
  abortion loading that file.  On some very specific specifier lines
  actually following CTags format, it could have led to loading a
  spurious tag instead.

Fixes #1814 and closes #1816.