keyfile: fill parse buffer in line sized chunks

When loading a keyfile the incoming bytes are fed
to a line buffer to get parsed each time a new line
is encountered.

The code that fills the line buffer does it inefficiently,
one byte at a time.

This commit changes that code to look ahead at the incoming
bytes for the next '\n' character and then fill the line buffer
all at once.

https://bugzilla.gnome.org/show_bug.cgi?id=650211