Add runtests.SH to the code that generates the targets sh and shextract.

The generated Makefile is functionally identical.
Programmatically generate dependencies for all *.SH files.

Previously dependencies for Makefile, config.h and makedepend were
explicitly coded into verbatim sections of Makefile.SH, with most of the
others being generated by makedepend.SH
Remove the now unused 'shlist' Makefile targets.
Add x2p/Makefile.SH to the *.SH files processed by the top level Makefile.

This makes the top level Makefile responsible for deleting x2p/Makefile.
With this, we can eliminate the 'sh' and 'shextract' from x2p's Makefile.
Remove rules to delete *.loadmap and *.prelmap files, generated on VM/ESA

These rules to clean up properly on VM/ESA were added in Oct 1998 by
perforce changes 1961, 1962 and 1963 in the confperl branch. VM/ESA support
was removed by commit 043fec90e88a2e23 in Aug 2012.
Simplify x2p's Makefile.

Eliminate plexe and plc, which relate to tests for the compiler and were
added as part of commit 52cebf5efc9883c7 in May 1988. The compiler was
removed by commit de1254415ffeb03b in Sept 2006.

The files a2p.man and s2p.man were removed with Perl 5.000.
The macros 'public', 'private' and 'util' mirror the structure of the top
level Makefile.SH. However, x2p only ever used 'public', so simplify things
by inlining the value of 'public' in the only place that uses it.
'addedbyconf' now only contains the value of 'plextract', so use the latter
directly in the one place that had used 'addedbyconf'.