fd/line: Add line buffered reader

This gives us similar functionality to std::getline but on stdplus::Fd
instances. Also more predictable behavior around newlines at the end of
a file.

Change-Id: I19039c15fa02019e4ad767ca8a1996a8417fdd86
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/meson.build b/test/meson.build
index 4ebf17f..a0be742 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -33,6 +33,7 @@
     'fd/managed',
     'fd/intf',
     'fd/impl',
+    'fd/line',
     'fd/mmap',
     'fd/mock',
     'fd/ops',