Update to 2.8.0-rc0

• Fix build with devel/ncurses installed [1], which erroneously redefines _POSIX_C_SOURCE as 200112L globally via its pkg-config file and hides strndup from cmus. A workaround for this was applied to cmus in DPorts commit d2907fe but it never made it back to FreeBSD.

Changes: https://github.com/cmus/cmus/releases/tag/v2.8.0-rc0

PR: 216325, 214475 [1]
Reported by: Oleg Gushchenkov <gor@clogic.com.ua> [1]
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D10242