base-files: rc.common: fix enable() return code and logic

base-files: rc.common: fix enable() return code and logic

In current state, if there is START but no STOP, enbale()
will return 1 (failure), which is wrong.
Moreover there is no need to check for START/STOP twice.
Instead, add err variable to save success state and
and return it's value.
Also eliminate the need to disable() by using 'ln -sf',
which will first delete the old symlink if one exists.

Changes from v1:
- fixed description

Signed-off-by: Roman Yeryomin <roman@advem.lv>