Fix sed functions 'i' and 'a' from discarding leading white space.

This appears to be implementation dependent but convenient and makes
our sed behave more like GNU sed.

Given that it is not the historic behavior, bump FreeBSD_version
should userland/ports somehow depend on it.

Obtained from: NetBSD (bin/49872)

Reviewed by: bdrewery
PR: 208554
Merge after: NEVER