getpagesize(3): add .Xr to sysconf(3)

POSIX deprecated getpagesize(3). The portable way to obtain the page
size is sysconf(_SC_PAGESIZE).

Reviewed by: cperciva (earlier), imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35352

(cherry picked from commit 49c937e7040c44afdcfccede57dcecaf9ff08ed6)