Make the size defaults and the chunk_size minimum 1000-based (closes #313)

The chunk_size and blob_size_limit defaults and the chunk_size minimum
were 1024-based, while the size parser reads 10MB as 10,000,000 bytes and
the docs and info use 1000-based units. So chunk_size: 1MB was rejected,
writing out a documented default changed it, and info showed an 11 GB
blob limit. They are now 1000-based like the parser. The examples below
the 1MB minimum in config.example.yml and the config init template are
replaced, and both now state the minimum.

Judgement call: s3.part_size keeps its 5MiB to 5GiB bounds, which are
S3's own limits, and MiB and GiB stay listed as accepted spellings.

Model: opus-5-5

config.example.yml

@@ -302,7 +302,8 @@ storage_url: "rclone://myremote/path/to/backups"
 
 # Average chunk size for content-defined chunking
 # Smaller chunks = better deduplication but more metadata
-# Supports: 10MB, 5M, 1GB, 500KB, 64MiB, etc.
+# Minimum 1MB
+# Supports: 10MB, 5M, 1GB, 64MiB, etc.
 # Default: 10MB
 #chunk_size: 10MB