Add shadow support

Packages describing “shadow” as local USE flag

Package “shadow” Flag Description
net-fs/netatalk Add shadow support
app-admin/puppet Enable shadow support
sci-physics/root Enable shadow authentication in rootd and proofd
net-dialup/freeradius-client Enable shadow password support
net-mail/tpop3d Enable /etc/shadow authentication
net-misc/dropbear Enable shadow password support

All packages providing a “shadow” USE flag (4)