[PATCH] Support using "all" for the db user in pg_ident.conf

While pg_hba.conf has supported the "all" keyword since a very long
time, pg_ident.conf doesn't have this same functionality. This changes
permission checking in pg_ident.conf to handle "all" differently from
any other value in the database-username column. If "all" is specified
and the system-user matches the identifier, then the user is allowed to
authenticate no matter what user it tries to authenticate as.

This change makes it much easier to have a certain database
administrator peer or cert authentication, that allows connecting as
any user. Without this change you would need to add a line to
pg_ident.conf for every user that is in the database.

In some small sense this is a breaking change if anyone is using "all"
as a user currently and has pg_ident.conf rules for it. This seems
unlikely, since "all" was already handled specially in pg_hb.conf.
Also it can easily be worked around by quoting the all token in
pg_ident.conf. As long as this is called out in the release notes
it seems okay to me. However, if others disagree there would
be the option of changing the token to "pg_all". Since any
pg_ prefixed users are reserved by postgres there can be no user.
For now I used "all" though to stay consistent with pg_hba.conf.