Description of problem:
Local authentication does not work with Amanda disk-based tape changer; using amrecover with "-o auth=local" fails to restore files.

Version-Release number of selected component (if applicable):
amanda-3.3.3-17.el7.x86_64

How reproducible:
Always

Steps to Reproduce:
1.
Configure Amanda to use a disk-based tape changer; e.g.:
define changer mychanger {
  tapedev "chg-disk:/path/to/dumps"
}
tapedev "mychanger"

2.
Run 
  amrecover DailySet1 -o auth=local
and attempt to extract a file from the chg-disk device.
3.

Actual results:
amrecover reports:
  Got no header and data from server, check in amidxtaped.*.debug and amandad.*.debug files on server

and the amidxtaped.*.debug file under /var/log/amanda/server/DailySet1 reports:
  amidxtaped: Insecure dependency in unlink while running setgid at /usr/lib64/perl5/vendor_perl/Amanda/Changer/disk.pm line 477.

Expected results:
The files should be successfully extracted.

Additional info:
Line 259 of ./amanda-3.3.3/common-src/local-security.c from amanda-3.3.3.tar.gz in the source RPM reads:
        setregid(uid, gid);

Clearly, this should instead be
        setregid(gid, gid);

Because the amandabackup user has uid=33 and gid=6(disk), amandad sets its credentials with gid=33 and egid=6; this triggers the Perl security check when amandad invokes amidxtaped.