Add plumbing to allow to push down an alias attribute to be used by
the memory cache.

Basic plumbing to allow passing an alias down to mmap cache functions.

This bumps the cache major number to 1 as this change is incompatible
with current clients.
The addiotinal key is used to allow name aliases to be added to user
and group entries. Aliases are a string that can be looked up via the
getpwname and getgrnam calls, and are useful to to fetch entries by
alternate names.
Currently only one, optional alias is allowed.

If a fatal EFAULT error is returned by the internal function that frees used
memory invalidate the whole cache and reinit it. This way we avoid further
corruption and insure clients see consistent data.

Also insure we use the right context in init() and we use talloc_zfree() in
reinit so that if the init() later fails we do not leave around a pointer
to free memory in the callers.

We should never try to invalidate an already invalid record as
internal pointers will not be consistent. Carefully test that the
record really is valid when we are fishing for free space, and
properly invalidate records or return a fatal error if something
goes wrong.
In order to make the code more robust always invalidate the whole
data space on initialization by setting all bits to 1, and make sure
to invalidate the whole last allocated slot by converting rec->len to
the number of slots instead of just the space used.

We were holding up slots when entries were invalidated directly an not through
our primitive garbage collection scheme.

Although it should enver happen that we pass in an invalid hash it
is always better to just not do anything than access memory ouf of
the hash table. It can lead to segfaults, or worse referencing
memory that should not be touched.

A recent patch introduced a glaring memory leak in the routines that clean up
memcache memory on initgroups calls.

The monitor sends calls different sbus methods to different responders.
Instead of including headers of the particular responders directly in
monitor, which breaks layering a little, create a common header file
that will be included from src/responder/common/

This is to allow to freely call mc functions even if initialization failed.
They will now gracefully fail instead of segfaulting.