#109 is_seekable() returns false for streams creted fmemopen

[Stefan Sauer]

For cases where we pass a buffer and buffer_size to open_read and have
fmemopen, is_seekable was returning false. The effect of this is that reading files and using format auto-detection fails when reading.
With this patch applied the implementation checks whether seeking actually works when files are not
regular files.

[Stefan Sauer]

Improved version that also handles fstat failing, the current code would read uninitialized memory.

[Mans Rullgard]

I have applied a simper version with just the fseek() test.