Refactor the Makefile so that the install target depends on install-all.

Also, add targets install_notify and install-notify, as synonyms for
no_install and no-install. Previously install, install-all and install_all
were all generated by the same rule.

These changes make future refactoring easier.
In Makefile.SH, use the long names for options passed to install{perl,man}

This makes it clearer what is going on, and enables a simplifying refactoring.
Refactor Makefile.SH to generate the install-* targets in a loop.

This slightly reduces the line count, merges five-fold duplication into one,
and works toward removing 5 recursive calls from Makefile to itself.
Inline the Makefile rule for extra.install into all 6 targets which use it.

This eliminates 6 recursive calls from Makefile to itself. As five of these
are generated from a loop in Makefile.SH, this isn't massive code
duplication. It's not obvious how to merge the sixth without increasing the
code complexity.

Additionally, if the user has specified extra modules to download and build
from CPAN using Configure's -Dextra, this avoids building perl, the non-XS
modules and the CPAN modules twice on make install.
Don't call CPAN to install any "extra" modules for the install-notify target.

Previously the Makefile's install-notify target built extra.install just like
the other 5 install targets. However, the other 5 actually install perl,
whereas install-notify (a.k.a. no-install) runs installperl and installman
with the --notify option (formerly -n), to cause them to print what they would
do but take no action. Hence if -Dextras was used, make no-install would
actually install the extra modules, but nothing else. This is inconsistent.
Merge the refactoring of the Makefile install targets into blead.