sed: use getline() instead of fgetln().

sed: use getline() instead of fgetln().

In BSD, fgetln() available in libc but in Illumos the Solaris port had to
include it internally. It also seems to have caused problems [1].

Aid portability by using getline() instead.

Reference:
https://www.illumos.org/issues/3820 [1]

Submitted by:	Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.com>
Reviewed by:	dds
MFC after:	2 weeks