Format:
maintain <spell name>
unmaintain <spell name>

The spell maintenance skill is used to keep certain spells up continuously,
so that they do not expire after a certain time.  Instead of needing to
recast the spell periodically, the spell saps a small amount of mana from
the caster for as long as the spell is up.  A spell maintained with the
'maintain' command can also be removed with the 'unmaintain' command.

Here is a list of well known spells that can be maintained:
armor                   detect invisibility     detect evil
infravision             sense life              fly
faerie fire             darken                  detect undead