man(1): Support custom sections

PR: 271830
Signed-off-by: Mohamed Akram <mohd.akram@outlook.com>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/764

(cherry picked from commit 8edb6fb572f29578877fdac3c30f718e589a0360 - needed for further bugfixes)