Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r246880:

Since clang 3.2 now has an option to suppress warnings about implicitly
promoted K&R parameters, remove the workarounds added for sendmail
components in r228558.

MFC r232263:

Define several extra macros in bsd.sys.mk and sys/conf/kern.pre.mk, to
get rid of testing explicitly for clang (using ${CC:T:Mclang}) in
individual Makefiles.

Instead, use the following extra macros, for use with clang:
- NO_WERROR.clang (disables -Werror)
- NO_WCAST_ALIGN.clang (disables -Wcast-align)
- NO_WFORMAT.clang (disables -Wformat and friends)
- CLANG_NO_IAS (disables integrated assembler)
- CLANG_OPT_SMALL (adds flags for extra small size optimizations)

As a side effect, this enables setting CC/CXX/CPP in src.conf instead of
make.conf! For clang, use the following:

CC=clang
CXX=clang++
CPP=clang-cpp

MFC r228558, r228585, r228588, r228652, r228707:

Unfortunately, clang gives warnings about sendmail code that cannot be
turned off yet. Since this is contrib code, and we don't really care
about the warnings, just make them non-fatal for now.

MFC r228590:

Fix typos in the comments about clang warnings in several
sendmail-related Makefiles.

Spotted by: arundel

MFC r228708:

In usr.bin/vacation/Makefile, fix a typo in the comment about clang
warnings.

Spotted by: arundel

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Make WARNS=6 the default for libexec/.

Just like bin/ and sbin/, I think setting WARNS to the highest value
possible will make it more attractive for people to fix warnings.

- The WARNS variable is set in the Makefile in the directory of the
application itself, making it more likely that it will be removed out
of curiosity to see what happens.
- New applications will most likely build with WARNS=6 out of the box,
because the author would more likely fix the warnings during
development than lower WARNS.

Unfortunately almost all apps in libexec require a lowered value of
WARNS.

Remove kludges intended to support src trees with partial obj trees.

Discussed with: ru

Remove MAINTAINER= lines from individual Makefiles in favor of the
MAINTAINER file (which already had entries for sendmail).

Update build infrastructure for sendmail 8.12.

- Backout botched attempt to intoduce MANSECT feature.
- MAN[1-9] -> MAN.

Set the default manual section for libexec/ to 8.

Add a MAINTAINER= line so people know who to blame

Style fixes

Give users a way to alter the sendmail (and related utilities) build
environment so they can enable functionality such as SASL, LDAP, Hesiod.

The rest of the changes needed to support the new version of sendmail (8.11.0).
Beyond changes to the build system, this includes fixing up the sample
freebsd.mc configuration for changes in defaults and syntax, removing
outdated documentation, and updating the release notes.

Build sendmail-8.9.1 smrsh