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/src/meson.build b/src/meson.build
index 34c3674..4364395 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -37,6 +37,7 @@
     'fd/create.cpp',
     'fd/dupable.cpp',
     'fd/impl.cpp',
+    'fd/line.cpp',
     'fd/managed.cpp',
     'fd/mmap.cpp',
     'fd/ops.cpp',