Explicit warning when root is needed to upgrade DB

[tigergao99]

Prints an explicit warning when errmsg has the substring "readonly".
Fixes #1771

[ceri]

Could this also check that pkg isn't running as root, otherwise it's possible that this may give a misleading error explanation (if the filesystem is read-only, for example).

[tigergao99]

Thanks for the catch. Now using geteuid() to check if user is root.

[bapt]

You should also check for "install as user" concept

[grahamperrin]

Context:

% pkg update -f
pkg: Insufficient privileges to update the repository catalogue.
% pkg query %v pkg
1.18.4
%