Fix some program names

Talk about pre-existing pag-screen sessions (not owl-screen) and
cont-renew-notify being missing (not cont-renew).
Use a sensible default for KRB5CCNAME

If KRB5CCNAME is unset, pag-screen now looks for tickets in the
location specified by 'klist'.

Fixes pag-screen #4.
Add renew-all

Run kinit -r7d for each principal with tickets in a DIR cache.
Handle empty KRB5CCNAME
Check for aklog before calling it
Support screen's -S and -c options

This allows specifying a custom session name and config file.