sysutils/dvd+rw-tools: Fix issue appending more than 25gb on BD-R DL discs

While burning ISO Multisession BD-R 50G growisofs reports no space left while
24G+ are available.

Backport a fix from Debian [1] to address the issue.

While I'm here, document description and origins of both patches [1][2] that
apply to same source file, for our future selves, and to avoid confusion
as both chunks reference the same variable.

[1] https://bugs.debian.org/615978
[2] https://bugs.debian.org/713016

PR: 230493
Submitted by: Masachika ISHIZUKA <ish amail plala or jp>
Approved by: portmgr (implicit, unmaintained port)
Obtained from: Debian [1]
MFH: 2018Q3