Use ASCII characters for box/line characters in frames.4th

Committed with changes to support the following from loader.conf(5):
+ console="vidconsole comconsole" (not just console="comconsole")
+ boot_serial="anything" (not just boot_serial="YES")
+ boot_multicons="anything" (unsupported in originally-submitted patch)

PR: conf/121064
Submitted by: koitsu
Reviewed by: gcooper, adrian (co-mentor)
Approved by: adrian (co-mentor)