rc.d/ddb: Really silently exit

The comment says “silently exit if ddb is not enabled”, but we'd exit
with an error message.

Note that I switched the sysctl variable used to test for the presence
of ddb from debug.ddb.scripting.scripts to d.d.s.script, which has a
smaller value if set.

While here, drop a pointless fork-exec, and use ${SYSCTL_N} for
consistency.

PR: 177217
MFC after: 1 week
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D57686

(cherry picked from commit b0d3a2889a1b8e4b73ad1f396884224e1442b214)