dirs: dbdir default must be based on sysconfdir.

dirs: dbdir default must be based on sysconfdir.

Some in-tree and out-of-tree code sets the OVS_SYSCONFDIR environment
variable to control where /etc files go (mostly for test purposes).  When
the database directory (dbdir) was split off from the sysconfdir, the
configure-time default continued to be based on the sysconfdir, but
overriding the sysconfdir at runtime with OVS_SYSCONFDIR didn't have any
effect on the dbdir, which caused a visible change in behavior for code
that set the OVS_SYSCONFDIR environment variable.  This commit reverts that
change in behavior, by basing the dbdir on OVS_SYSCONFDIR if that
environment variable is set (but the OVS_DBDIR environment variable is
not).

Signed-off-by: Ben Pfaff <blp@nicira.com>