Clean up lilypond-book snippet formatting and file name hashing

Clean up lilypond-book snippet formatting and file name hashing

process_snippets(): eliminate duplicates in snippet lists written in
snippet-names-*.ly, which fixes issue #867.

LilyPondSnippet.get_checksum(): reintroduce in hashing fragment
options that matter for processing by lilypond, which had be neglected
since commit 4c5a581ca25398669b9ecbc7a606febb09e60214.

LilyPondSnippet.basename(): remove duplicated truncation of hash hex
value.

Ly snippet formatting: sort fragment options for more predictable and
repetable output, and don't write fragment options that don't have an
impact of lilypond processing.

Fix a few other nitpicks (e.g. comments).