Remove subversion keyword substitutions

Remove subversion "Id" keyword substitutions from source files as
they're not necessary and are not supported by Git.
Add distribution tarballs to .gitignore
Fix format read test error reporting
Fix spelling of "retention"
Add hidrd_buf_own

Add hidrd_buf_own - a function initializing a buffer, taking ownership
of a dynamically-allocated memory chunk.
Replace hidrd_buf_detach with hidrd_buf_disown

Replace hidrd_buf_detach function with a more appropriately named
hidrd_buf_disown, also returning the buffer allocated size.
Add hidrd_buf_add_char

Add hidrd_buf_add_char - a function adding a single character to a
buffer. Use it instead of hidrd_buf_add_span, where appropriate.
Check hidrd_buf_add_char result

Add checking of hidrd_buf_add_char invocation result, in two places,
where it was missing.
Add hidrd_buf_del

Add hidrd_buf_del - a function removing specified number of bytes from
the end of a buffer's contents.
Add hex dump format support

Add support for "hex" - a hex dump format with both input and output
support. Fixes #2 "Add an hex dump input format".