…ize.

Note the md4sum blocksize is 64, so using 128 ensures it is an integer
multiple of the strongsum blocksize for both kinds of strongsum.

This makes calculating both blake2b and md4sum strongsums a tiny bit faster,
since they don't need to add a padded tail block when finalizing the sum.

Add suggestion of strong_len=20 as a good compromize between safety against
attacks and signature size.

Add suggestion to use rs_sig_size() and argument defaults to rs_sig_file().

Change the titles and refer to the api's as "Streaming API", "Callback API"
and "Whole-file API".

Add instructions about using rs_sig_args() and rs_build_hash_table().

Remove instructions about setting job->sig_file_bytes to pre-allocate the
signature because this is not publicly exposed... the rs_job_t structure is
not public.

All other references use '\' as the doxygen escape, so be consistent.

This updates a heap of comments and removes some obsolete settings.

This means we use an updated doxygen to match the updated config.

This reverts commit 5cd40a7.

This didn't help. The bionic version of doxygen has a bug and emits even more
warnings that the xenial version.