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ypldap: Remove set but unused variables.

Some of these were reported by GCC, others reported by clang.

Increase YPMAXRECORD to 16M to be compatible with Linux.

Since YP protocol definition uses the constant to declare
variable-size opaque byte strings, the change should be binary
compatible with existing installations which do not expose keys or
values larger than 1024 bytes.

All uses of local variables with YPMAXRECORD sizes were removed to
avoid insane stack use. On the other hand, variables with static
lifetime should be fine and only result in increased VA use.

Glibc made same change, increasing the allowed length for keys and
values in YP to 16M, in 2013.

Reviewed by: markj
Discussed with: ian
Sponsored by: Mellanox Technologies
MFC after: 3 weeks
Differential revision: https://reviews.freebsd.org/D20900

Use memset(3) instead of bzero(3).

Obtained from: OpenBSD (r1.12, r1.20, r1.18 and r1.37)

Use nitems instead of 'sizeof(mapnames) / sizeof(mapnames[0]'.

Set argument encode/result decode call backs for 'maplist' and 'all'.

Note: Listing a map is still not fully implemented.

Obtained from: OpenBSD (r1.16, r1.17)

Implement the 'master' request.

Obtained from: OpenBSD (r1.15)

Switch from FD_SETSIZE to getdtablesize(2) as it can make the FD to be
tunable. Also it gets more close with the original implementation from
OpenBSD.

Requested by: rodrigc
Approved by: rodrigc (mentor)
Differential Revision: https://reviews.freebsd.org/D4970

ypldap(8) is a feature ready to be used to translate nis(8) database to ldap(3).

This commit, fix a core dump on ypldap(8) related with memory allocation.
Also an example of how to set the ypldap.conf(5) properly is added to
examples files.

A new user _ypldap is required to be able to run ypldap(8) as well as
in a chroot mode.

Reviewed by: rodrigc (mentor), bjk
Approved by: bapt (mentor)
Relnotes: Yes
Sponsored by: gandi.net
Differential Revision: https://reviews.freebsd.org/D4744

Replace __svc_fdset with svc_fdset.

FreeBSD lacks __svc_fdset and __svc_fdsetsize.

Add include of sys/param.h for MAXHOSTNAMELEN

Import ypldap from OpenBSD.

ypldap -- Intended to be a drop-in replacement for ypserv, gluing in a
LDAP directory and thus providing support for users and groups stored in
LDAP for the get{pw,gr}ent family of functions.