test: replace notmuch_passwd_sanitize() with _libconfig_sanitize()

test: replace notmuch_passwd_sanitize() with _libconfig_sanitize()

notmuch_passwd_sanitize() in test-lib.sh is too generic, it cannot
work in many cases...

The more specific version _libconfig_sanitize() replaces it in
T590-libconfig.sh and the code that uses it is modified to output
the keys (ascending numbers printed in hex) so the sanitizer knows
what to sanitize in which lines...

"@" + fqdn -> "@FQDN" replacement is used as fqdn could
-- in theory -- be substring of 'USERNAME'.

'user -> 'USER_FULL_NAME replacement to work in cases where user
is empty -- as only first ' is replaced that works as expected.

In addition to ".(none)" now also ".localdomain" is filtered from
USERNAME@FQDN.

/dev/fd/{n} is not defined in posix, but it is portable enough
(if it weren't it is easy to fix -- now code looks clearer).