config: drop dependency on libsbuf

Use an std::stringstream instead. get_word() and get_quoted_word() both
return a buffer that's presumed to not need release, so solve this by
returning a new special configword type that holds a string or eof/eol
state. This cleans up caller checking for EOF/EOL to make it more
explicit what they're doing, at least in the EOL cases which previously
checked for NULL.

Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D38276