sysv-generator: use generator_add_symlink()

sysv-generator: use generator_add_symlink()

generator_add_symlink() is extended to ignore EEXIST. This should be fine
for all existing callers.

There's a small difference in behaviour when adding symlinks in sysv-generator:
the message is more generic and does not include ", ignored". But creation of
symlinks shouldn't ever fail except if things are very wrong, so in practice
this shouldn't matter.

Test needed updating: os.path.exists(os.readlink(link)) only works if the link
is absolute (or if we are in the right directory). Let's just use
os.path.exists(link), which properly tests that the symlink target exists.