Add error context callback when tokenizing authentication files

Add error context callback when tokenizing authentication files

The parsing of the authentication files for HBA and ident entries
happens in two phases:
- Tokenization of the files, creating a list of TokenizedAuthLines.
- Validation of the HBA and ident entries, building a set of HbaLines or
IdentLines.

The second phase doing the validation provides already some error
context about the configuration file and the line where a problem
happens, but there is no such information in the first phase when
tokenizing the files.  This commit adds an ErrorContextCallback in
tokenize_auth_file(), with a context made of the line number and the
configuration file name involved in a problem.  This is useful for files
included in an HBA file for user and database lists, and it will become
much more handy to track problems for files included via a potential
@include[_dir,_if_exists].

The error context is registered so as the full chain of events is
reported when using cascaded inclusions when for example
tokenize_auth_file() recurses over itself on new files, displaying one
context line for each file gone through when tokenizing things.

Author: Michael Paquier
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/Y2xUBJ+S+Z0zbxRW@paquier.xyz