Allow the use of the 'depends_on provider' directive when dtrace(1)
isn't running as root. This is done by looking up providers in the
debug.dtrace.providers sysctl before trying an ioctl.
Don't call dt_provider_lookup() twice if we failed to read the sysctl.
Merge branch 'dtrace-depends-on' into markj

Conflicts:
	cddl/contrib/opensolaris/lib/libdtrace/common/dt_pragma.c