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 95ed3294 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