BBLog: improve sysctl variables

Correct the format in sysctl net.inet.tcp.bb.disable_all and
sysctl net.inet.tcp.bb.log_auto_all.
Correct the format and the description in
net.inet.tcp.bb.log_auto_mode.

Reviewed by: rscheff, tuexen
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D37776

(cherry picked from commit 57cc27a3325af74553e852851d5e84e308ae79f4)