* Change the code examples in libdwarf's manual page set to use the
`EXIT_FAILURE` symbol instead of a numeric return code.
* Style fixes: use `.Bd -offset indent`.
Revise the `dwarf_undef_macro.3` manual page:
* Document the special meaning of a value of zero for the `lineno` parameter
* Add an EXAMPLE section showing a simple usage example.
* Improve a few sentences.
