Switch the default pager from "more -s" to "less".

Switch the default pager from "more -s" to "less".

POSIX explicitly allows using a different default pager if that is
documented.  Nowadays, the pager provided in most operating systems
is less(1).  Our man(1) implementation uses less(1) features that
traditional more(1) did not provide, in particular tagging.  Besides,
as noted by deraadt@, the user interface of less(1) is slightly
more refined and preferable over the user inferface of more(1).
This switch was originally suggested by Ian Ropers.

In ./configure, test whether less(1) is available.  If not, fall
back to more(1).  In ./configure.local, support overriding the
automatic test by setting BINM_PAGER.

As explained by jmc@ and deraadt@, the -s flag was added a very
long time ago when an antique version of groff(1) had an annoying
bug in terminal output that would randomly display blank lines in
the middle of pages.  Clearly, -s has no longer been needed for
many years, so drop it from the default pager invocation.

OK deraadt@ jmc@ martijn@ job@ on the OpenBSD version of this patch.