Merge branch 'PR/build-sys-libeconf-split' of https://github.com/karelzak/util-linux...

* 'PR/build-sys-libeconf-split' of https://github.com/karelzak/util-linux-work:
  meson: check slang headers only when slang library is found
  meson: rename logindefs_c to lib_common_logindefs
  meson: split shells.c out of lib_common into lib_common_shells
  build-sys: drop libcommon_shells from binaries that only need ul_default_shell
  lib: split ul_default_shell() from shells.c into default_shell.c