build-sys: add --enable-usrdir-path

build-sys: add --enable-usrdir-path

The directories /{sbin,bin} are symliks to /usr/{sbin,bin} on many
systems. This patch add new ./configure option to remove the non-usr
paths from the default $PATH environment variable.

The default $PATH is hardcoded in login(1) and can be overwritten
by /etc/login.defs.

default:

./test_pathnames | grep DEFPATH
       _PATH_DEFPATH /usr/local/bin:/bin:/usr/bin
  _PATH_DEFPATH_ROOT /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

--enable-usrdir-path:

./test_pathnames | grep DEFPATH
       _PATH_DEFPATH /usr/local/bin:/usr/bin
  _PATH_DEFPATH_ROOT /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin

This patch does not modify install paths, you still have to care about
--{bin,lib}dir configure options.

Signed-off-by: Karel Zak <kzak@redhat.com>