Proper field initialization.

gcc -Wmissing-field-initializers noticed several struct initializations
that didn't explicitly list all fields. They were of no consequence because
they were for static data, so they were initialized properly.
Also, changed the initialization of global.c_ovtxt from "" to NULL because
free_queue () freed it if non-NULL, though I think the "" was always
overwritten.

Pulled in from nmh. Thanks to David Levine.