This handles integer-valued vars in a singleton struct, and exports functions
for incrementing, setting and getting those vars, as well as rendering all the
vars in a standard format.

Demonstrate the use of the exvar package in the http/triv server.

R=dcross,r
APPROVED=r
DELTA=122  (122 added, 0 deleted, 0 changed)
OCL=27617
CL=27622