Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Make the directory mapping functionality, which was previously only
available in 32-bit compatibility mode, unconditional.

Overhaul the man page, which had evolved more by accretion than by design.

Approved by: re (gjb)
MFC after: 3 weeks

Description from Dan:

Another handy libmap patch. Lets you do stuff like this:

LD_LIBMAP="libpthread.so.1=libthr.so.1" mythreadedapp

If you already have a program-specific override in libmap.conf, note
that you must use a program-specific override in LD_LIBMAP:

LD_LIBMAP="[mythreadedapp],libpthread.so.1=libthr.so.1" mythreadedapp

PR: bin/74471
Submitted by: Dan Nelson <dnelson AT allantgroup.com>
MFC after: 2 weeks

Add initial support for compiling a special 32 bit version of
ld-elf.so.1 on 64 bit systems. Most of this involves using alternate
paths, environment variables and diagnostic messages.

The build glue is seperate.

Change libmap.c:lm_init() to return a status value; 0 for success
(libmap available) and 1 for failure. Assign this return to the
global 'libmap_disable' variable in rtld.c.

This totally prevents any libmap functions from being called after
lm_init() if no config file is present.

Dynamic object dependency mapping: libmap.

This is an optional feature, disabled by default.

This will be useful to people testing the various POSIX threading
libraries under -CURRENT but can easily serve other needs.