Change: move iterators and finders to dedicated user files

[mattmundell]

What

Move user iteration and user finding out of manage_sql.c.

Why

Reduces size of manage_sql.c. Groups the resources into files more consistently.

References

Follows /pull/2708.

Testing

I ran GET_USERS, comparing output with main.

# on main
o m m '<get_users/>' > /tmp/u-main
# with pr
o m m '<get_users/>' > /tmp/u-pr
diff /tmp/u-main /tmp/u-pr