nl: Increase receive buffer size to 4 pages

Assuming that the kernel does not send more than a page is no longer valid,
and enabling MSG_PEEK'ing by default to figure out the exact message buffer
requirements can have a negative influence on the performance of existing
applications. Bumping the default receive buffer space to 4 pages seems
a sane default.

Signed-off-by: Thomas Graf <tgraf@suug.ch>