docs: add a prebuilt manpage to the distribution

Add a prebuilt manpage for nfs-ls and change the makefiles to not build the
manpage by default. This is because the manpages change rarely and this removes
the need for an internet connection when building the library.
(previously building the library required downloading a template from
sourceforge)

Update the README and mention that you now need to manually rebuild the
manpages if the manpage sources change.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>