libpathconv: Fix mandoc warnings in abs2rel(3) and rel2abs(3)

• cannot parse date, using it verbatim: Dec 15, 1997"
• sections out of conventional order: Sh SEE ALSO
• possible typo in section name: Sh EXAMPLE instead of EXAMPLES
• AUTHORS section without An macro

(cherry picked from commit 094517119c62c23369d545a7475ae982d86330a3)