sysv-generator: Handle .sh suffixes when translating Provides:

When deciding whether the provided name equals the file name in
sysv_translate_facility(), also consider them equal if the file name has a
".sh" suffix.

This was uncovered by commit b7e71846 which then created a symlink
"<name>.service" to itself for ".sh" suffixed init.d scripts.

For additional robustness, refuse to create symlinks to itself in add_alias().

Add test case which reproduces the bug.

https://bugs.debian.org/775889