cksum/hashsum: Support for commented lines in checksum files

[RenjiSann]

As #6603 implicitly implemented commented line support, I moved it into its own PR, with proper testing.

TL;DR:
This change make the checksum verification system ignore lines starting with #, treating them as empty lines.

Example:

$ echo 'foo-content' > foo
$ cksum --algo=sha1 foo | tee CHECKSUM
SHA1 (foo) = 058ab38dd3603703b3a7063cf95dc51a4286b6fe 

$ echo '# Comment' >> CHECKSUM 
$ cat CHECKSUM
SHA1 (foo) = 058ab38dd3603703b3a7063cf95dc51a4286b6fe
# Comment

$ cksum --check CHECKSUM
foo: OK

The changes similarly apply to hashsum and its derivatives.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/timeout/timeout is no longer failing!

[RenjiSann]

@BenWiederhake

[cakebaker]

Looks good, great to see more tests :)

[cakebaker]

Thanks :)