sysusers: move global variables into a Context object

sysusers: move global variables into a Context object

Our coding style says static variables suck except for very special
cases, i.e. things like the log level or very per-process stuff, such as
parsed version of cmdline args and such. sysusers departed from that as
one of the very few exceptions in our codebases: it keeps its
operational state in global variables.

Address that. Introduce a Context object that carries the fields that so
far have been global, and pass it around as needed.

This has the nice effect that state and configuration is clearly
separated in code, and we can very clearly see which functions mangle
state and which ones do not.

No actual codeflow changes, just refactoring.