walmethods.c/h: Make Walfile a struct, rather than a void *

This makes the curent file position and pathname visible in a generic
way, so we no longer need current_walfile_name global variable or the
get_current_pos() method. Since that purported to be able to fail but
never actually did, this also lets us get rid of some unnecessary
error-handling code.

One risk of this change is that the get_current_pos() method
previously cleared the error indicator, and that will no longer happen
with the new approach. I looked for a way that this could cause problems
and did not find one.

The previous code was confused about whether "Walfile" was the
implementation-dependent structure representing a WAL file or
whether it was a pointer to that stucture. Some of the code used it
one way, and some in the other. The compiler tolerated that because
void * is interchangeable with void **, but now that Walfile is a
struct, it's necessary to be consistent. Hence, some references to
"Walfile" have been converted to "Walfile *".

Discussion: http://postgr.es/m/CA+TgmoZS0Kw98fOoAcGz8B9iDhdqB4Be4e=vDZaJZ5A-xMYBqA@mail.gmail.com

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