Help: Add install() command document section headers

Use section headers instead of horizontal dividers so that one may link
to the sections.
cmInstallTargetGenerator: Drop default constructor arguments

They are used only in cmLocalGenerator::GenerateTargetInstallRules.
Move the defaults to a local helper where the context justifies their
values.
file(INSTALL): Do not pre-create DESTINATION for DIRECTORY

When installing a DIRECTORY, do not pre-create the DESTINATION.  The
cmFileCopier::InstallDirectory method will create the directory anyway.
Give it a chance to detect whether the directory already exists or not.
file(INSTALL): Report existing DIRECTORY as Up-to-date

Teach cmFileCopier::InstallDirectory to detect whether the destination
directory exists.  If so, report it as "Up-to-date" instead of
"Installing".  This resolves message asymmetry with file installations.

Extend the RunCMake.file and RunCMake.install tests to check the
installation output on both the first and second run.

Suggested-by: J Decker <d3ck0r@gmail.com>
file(INSTALL): Add undocumented options to control output verbosity

Create options "MESSAGE_ALWAYS", "MESSAGE_LAZY", and "MESSAGE_NEVER" to
specify whether to print the "Installing" and "Up-to-date" messages.
Extend the RunCMake.file test with cases covering these options.
install: Thread message level setting through internal API

Create a cmInstallGenerator::MessageLevel enumeration for future use in
specifying install message verbosity.  Thread values of the type through
constructors and save the value as a member of cmInstallGenerator.
Use only a "MessageDefault" value for now.
install: Add CMAKE_INSTALL_MESSAGE variable (#13761)

Create a variable to allow users to control which installation
messages are printed.  In particular, provide a "LAZY" setting
that prints "Installing" messages but not "Up-to-date" messages.
This is desirable for incremental re-installations.

Suggested-by: J Decker <d3ck0r@gmail.com>
install(DIRECTORY): Add MESSAGE_NEVER option to avoid output (#13761)

Installing large directories, e.g., the output of a doxygen run, prints
one line per file resulting in too much noise in the build output.  Add
an option to the install(DIRECTORY) command to not print anything upon
make install.

Extend the RunCMake.install test with cases covering MESSAGE_NEVER
behavior of the install(DIRECTORY) command.

Suggested-by: Stefan Eilemann <Stefan.Eilemann@epfl.ch>
Merge topic 'install-messages' into next

d19b64d6 install(DIRECTORY): Add MESSAGE_NEVER option to avoid output (#13761)
c9568de5 install: Add CMAKE_INSTALL_MESSAGE variable (#13761)
ec7cf7ea install: Thread message level setting through internal API
abebcd23 file(INSTALL): Add undocumented options to control output verbosity
464567a5 file(INSTALL): Report existing DIRECTORY as Up-to-date
f701b0b7 file(INSTALL): Do not pre-create DESTINATION for DIRECTORY
f0a01962 cmInstallTargetGenerator: Drop default constructor arguments
67815894 Help: Add install() command document section headers