Add a default flavor to an export's e_secinfo list

The list of security flavors that mountd allows for the NFSv4
pseudo-fs is constructed from the union of flavors of all current
exports.

exports(5) documents that the default security flavor for an
export, if "sec=" is not specified, is "sys".  Suppose
/etc/exports contains:

/a  *(rw)
/b  *(rw,sec=krb5:krb5i:krb5p)

The resulting security flavor list for the pseudo-fs is missing
"sec=sys".  /proc/net/rpc/nfsd.export/content contains:

/a  *(rw,root_squash,sync,wdelay,no_subtree_check,
        uuid=095c95bc:08e4407a:91ab8601:05fe0bbf)
/b  *(rw,root_squash,sync,wdelay,no_subtree_check,
        uuid=2a6fe811:0cf044a7:8fc75ebe:65180068,
        sec=390003:390004:390005)
/   *(ro,root_squash,sync,no_wdelay,v4root,fsid=0,
        uuid=2a6fe811:0cf044a7:8fc75ebe:65180068,
        sec=390003:390004:390005)

The root entry is not correct, as there does exist an export whose
unspecified default security flavor is "sys".  The security settings
on the root cause sec=sys mount attempts to be incorrectly rejected.

The reason is that when the line in /etc/exports for "/a" is parsed,
the e_secinfo list for that exportent is left empty.  Thus the union
of e_secinfo lists created by set_pseudofs_security() is
"krb5:krb5i:krb5p".

I fixed this by ensuring that if no "sec=" option is specified for
an export, its e_secinfo list gets at least an entry for AUTH_UNIX.

[ Yes, we could make the security flavors allowed for the pseudo-fs
a fixed list of all flavors the server supports.  That becomes
complicated by the special meaning of AUTH_NULL, and we still have
to check /etc/exports for whether Kerberos flavors should be listed.
I opted for a simple approach for now. ]

Acked-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>