lei: show {pct} and {oid} in From_ lines and filenames

lei: show {pct} and {oid} in From_ lines and filenames

From_ lines are shown when mbox* variants are output to stdout,
making {oid} and {pct} information visible without risking being
propagated to other importer processes if they were in
lei-specific X-* headers.

Maildirs already had OIDs in the filename, now they gain Xapian
{pct} in case anybody cares.