Ensure .CURDIR contains an absolute path without a trailing slash.

Failed builds can occur when PORTSDIR is a symbolic link, or with

make -C /usr/ports/category/port/

PR: 221296
Reported by: yasu@utahime.org, rum1cro@yandex.ru
Reviewed by: bdrewery, sjg
Approved by: portmgr (bdrewery)
Differential Revision: https://reviews.freebsd.org/D11934