sysv-generator: handle Provides: for non-virtual facility names

The list of provided facility names as specified via Provides: in the
LSB header was originally implemented by adding those facilities to the
Names= property via unit_add_name().

In commit 95ed3294c6 the internal SysV
support was replaced by a generator and support for parsing the Names=
option had been removed from the unit file parsing in v186.
As a result, Provides: for non-virtual facility was dropped when
introducing the sysv-generator.

Since quite a few SysV init scripts still use that functionality (at
least in distros like Debian which have a large body of SysV init
scripts), add back support by making those facility names available via
symlinks to the unit filename to ensure correct orderings between
SysV init scripts which use those facility names.

Bug-Debian: https://bugs.debian.org/774335
(cherry picked from commit b7e7184634)

src/sysv-generator/sysv-generator.c

@@ -112,6 +112,27 @@ static int add_symlink(const char *service, const char *where) {
         return 1;
 }
 
+static int add_alias(const char *service, const char *alias) {
+        _cleanup_free_ char *link = NULL;
+        int r;
+
+        assert(service);
+        assert(alias);
+
+        link = strjoin(arg_dest, "/", alias, NULL);
+        if (!link)
+                return log_oom();
+
+        r = symlink(service, link);
+        if (r < 0) {
+                if (errno == EEXIST)
+                        return 0;
+                return -errno;
+        }
+
+        return 1;
+}
+
 static int generate_unit_file(SysvStub *s) {
         char **p;
         _cleanup_fclose_ FILE *f = NULL;
@@ -462,7 +483,9 @@ static int load_sysv(SysvStub *s) {
                                         if (r == 0)
                                                 continue;
 
-                                        if (unit_name_to_type(m) != UNIT_SERVICE) {
+                                        if (unit_name_to_type(m) == UNIT_SERVICE) {
+                                                r = add_alias(s->name, m);
+                                        } else {
                                                 /* NB: SysV targets
                                                  * which are provided
                                                  * by a service are