Skip the format specifier when loading a global tag file
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.