check curses-libs by configure

check curses-libs by configure

Presently, the used curses-library are determined by OS.
The leads to a build error when no curses-headers are installed.
Use configure to test if curses.h or ncurses.h is present.
ncurses takes precedence over curses.h.
If neither the curses- nor ncurses-libs are available, do not build
afsmonitor and scout.
A summary at the end of the configure should make this clear to
everyone.
The variable TXLIBS has been renamed to LIB_curses.

Change-Id: I3f661e0b6199be41818dc22149b061e3d599b77f
Reviewed-on: http://gerrit.openafs.org/3345
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>