This makes it easier to specify values taken directly from WAL file
names.
The option parsing is arranged in the style of option_parse_int() (but
we need to parse unsigned int), to allow future refactoring in the
same manner.
Reviewed-by: Sébastien Lardière <sebastien@lardiere.net>
Discussion: https://www.postgresql.org/message-id/flat/8fef346e-2541-76c3-d768-6536ae052993@lardiere.net
4c8044c044