pgbench: Increase maximum size of log filename from 64 to MAXPGPATH.

Commit 41124a91 allowed the
transaction log file prefix to be changed but left in place the
existing 64-character limit on the total length of a log file name.
It's possible that could be inconvenient for somebody, so increase the
limit to MAXPGPATH, which ought to be enough for anybody.

Per a suggestion from Tom Lane.