Change hammer_str_to_tid() and its callers to restrict the format of

Change hammer_str_to_tid() and its callers to restrict the format of
strings after the '@@' portion of the filename.  Unknown formats are
simply passed through, so for example you can create a file called 'x@@b'
without it thinking it is 'x' with a @@b transaction id extension.

The formats allowed are:

    @@0x%016llx[:%05d]
    @@-1[:%05d]

Any other format is passed through and not treated specially.

Reported-by: "Steve O'Hara-Smith" <steve@sohara.org>