sys-libs/glibc: guard against rendering system unusable, bug #627338

sys-libs/glibc: guard against rendering system unusable, bug #627338

For cases like FEATURES=config-protect-if-modified
or custom /etc/nsswitch.conf with only 'compat' resolver
and external providers glibc upgrade will break local
user resolution when switching from older glibc.

Bad /etc/nsswitch.conf:
    passwd:      compat
    shadow:      compat
    group:       compat

Good /etc/nsswitch.conf:
    passwd:      files
    shadow:      files
    group:       files

Ok /etc/nsswitch.conf:
    passwd:     compat files
    shadow:     compat files
    group:      compat files

More up-to-date doc:
    https://wiki.gentoo.org/wiki/Project:Toolchain/nsswitch.conf_in_glibc-2.26

Bug: https://bugs.gentoo.org/627338
Package-Manager: Portage-2.3.13, Repoman-2.3.4