Fix global path substitutions

[gl11tchy]

Fixes #675.

Summary

• Apply a global substitution rule repeatedly against the remaining pathname.
• Advance after zero-length matches so a global rule cannot retry the same position forever.
• Add a regression test for -s /abc/ABC/gp replacing every match in a pathname.

Testing

• env N=8 VERBOSE=1 ./tests/test_tarsnap.sh .
• env VERBOSE=1 ./tests/test_tarsnap.sh . (04-c-d-real-keyfile skipped; no TARSNAP_TEST_KEYFILE configured)