[arch-dev-public] glibc 2.27 based toolchain in [staging]

[Bartłomiej Piotrowski]

New draft that includes pam_unix2 deprecation.

Title: glibc 2.27-2 and pam 1.3.0-2 may require manual intervention

The new version of glibc removes support for NIS and NIS+. The default
`/etc/nsswitch.conf` file provided by `filesystem` package already
reflects this change. Please make sure to merge pacnew file if it exists
prior to upgrade.

NIS functionality can still be enabled by installing `libnss_nis`
package. There is no replacement for NIS+ in the official repositories.

`pam 1.3.0-2` no longer ships pam_unix2 module and `pam_unix_*.so`
compatibility symlinks. Before upgrading, review PAM configuration files
in the `/etc/pam.d` directory and replace removed modules with
`pam_unix.so`. Users of pam_unix2 should also reset their passwords
after such change. Defaults provided by `pambase` package do not need
any modifications.