fs: fix durations like 1d12h being rejected - #9986

What does this change do?

The docs describe a duration as "a, possibly signed, sequence of decimal numbers, each with optional fraction, and each with a unit suffix", with d, w, M and y in the list of units. A sequence that uses one of those doesn't parse:

$ rclone lsf --max-age 1d12h /tmp/x
Error: invalid argument "1d12h" for "--max-age" flag: parsing "1d12h" as fs.Duration failed: parsing time "1d12h" as "2006-01-02": cannot parse "1d12h" as "2006"

time.ParseDuration handles 2h45m and parseDurationSuffixes handles a single number with one suffix like 1.5d, so anything mixing the two falls through to date parsing and fails with a confusing message.

This adds a step between the two that parses such a sequence, using ageSuffixes for d/w/M/y and time.ParseDuration for everything else, and rejects a total too long to fit in a time.Duration. Everything that parsed before takes the same path as before. I've also added 1d12h to the examples in the docs.

Tests: new cases in TestParseDuration (1d12h, 1w2d, 1.5d12h, -1d12h, plus malformed and overflowing sequences), which fail on master. make quicktest matches master (cmd/gitannex fails on both, no git-annex here) and golangci-lint run ./fs/ reports 0 issues. With the built binary, --max-age 1d12h lists a file touched 30h ago and skips one touched 48h ago.

Linked issue

None.

Checklist

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