Use the new internal SSSD errors, to simplify error handling.
Instad of having up to 3 different errors to care about (system, krb5 and
pam_status), collapse all error reporting into one error type mapped on
errno_t.

The returned error can contain either SSSD internal errors, kerberos errors
or system errors, they all use differen number spaces so there is no overlap
and tey can be safely merged.

This means the errors bein sent from the child to the parent is not a pam
status error message anymore. The parent properly deals with that.

Also not that this patch removes returning SSS_PAM_SYSTEM_INFO from the
krb5_child for kerberos errors as all it was doing was simply to make the
parent emit the same debug log already emitted by the child, and the code
is simpler if we do not do that.

EIO has always been an odd match, but was used as an error to indicate
that something had gone wrong internally before we had specific SSSD
errors available. Use ERR_INTERNAL instead going forward.

This prevents reportin false errors when internal functions return
a generic EINVAL or EACCES that should just be treated as internal
errors.

This code adds a new range of error codes specific to SSSD,
It also provides helper functions to print out error defintions
like you can do with system error messages and the strerror() function.

The sss_strerror() function can accept both the new sssd errors and
system errno_t errors falling back to the system strerror() if the error
code provide is not a valid SSSD error code.

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.

The aim of this refactoring is to make the code readable and understandable.
This code has grown organically over time and has becomed confused and
baroque enough that understanding it's very simple flow had become very
complex for the uninitiated. Complex flows easily hide nasty bugs.

Improvements:
- Remove dead/unused data storage
- Fix and simplify talloc hierarchy, use a memory context (kr) for the
whole code and allocate kr->pd where it is filled up.
- Rename some functions to create a better name space (easier for
searching fucntions across the tree)
- Streamline setup function, by spliting out fast setup in a subroutine.
- Avoid confusing indirection in executng actual functions by not
using the krb5_req child_req member.
- Make main() flow s now simmetric, send abck data from the main function
instead of delegating a reply to every inner function that implements a
command.

Now the flow is evident from the main function:
1. read request
2. setup data
3. execute command
4. send reply back

Use the standard 'done' label for exceptions.