To Done:
Observed brokenness:
[D] root ownership of symlinks in /p4/1. Deferred to SDP-391.
[X] firewall ports wrong for p4broker. Fixed.
[X] Need to add port for broker in reset_sdp.cfg. Fixed.
[X] firewall code doesn't work. Fixed.

To Do:
[ ] Install active ntp (network time protocol) service?
[ ] Set standard time zone?
[ ] Set standard host name? [ Yes for cloud deployments, maybe/maybe not for on-prem. Make optional.]
[ ] Setup SSH keys for perforce user (after host name change).
[ ] Add Interactive mode.
[ ] Extend -X to undo:
   [X] Firewall stuff.
   [ ] Systemd and SysV init stuff.
   [ ] Perforce Package Repository connection.
[-] Do 'yum install' by default; don't skip it with '-fast'.

To Ponder:
[X] Maybe: Add a general 'apt-get update' for Ubuntu before and after repo
    changes. (Definitely a good idea; just need to explore whether it's
    already working like that).
[-] Extend -X to undo changes made by os_tweaks.sh. (Nah, this would require
    reliable tracking of what was changed with os_tweaks.sh between calls to
    reset_sdp.sh to do something of limited value. The os_tweaks changes are
    always good changes, not to be undone.)