Store IdentLine->pg_user as an AuthToken

Store IdentLine->pg_user as an AuthToken

While system_user was stored as an AuthToken in IdentLine, pg_user was
stored as a plain string.  This commit changes the code as we start
storing pg_user as an AuthToken too.

This does not have any functional changes, as all the operations on
pg_user only use the string from the AuthToken.  There is no regexp
compiled and no check based on its quoting, yet.  This is in preparation
of more features that intend to extend its capabilities, like support
for regexps and group membership.

Author: Jelte Fennema
Discussion: https://postgr.es/m/CAGECzQRNow4MwkBjgPxywXdJU_K3a9+Pm78JB7De3yQwwkTDew@mail.gmail.com