Fixed src/README.txt. Portable C Installation instructions.

Fixed src/README.txt.  Portable C Installation instructions.

* doc/user-manual/user.texinfo: Added node "Portable C Installation".
Added "Type and range checking" describing the no-type-checks and
no-range-checks declarations.

* src/README.txt: Added a preface for the newbie that runs across this
file in a binary distribution.  Removed "CVS".  Added "portable C
distribution" and "MIT_SCHEME_EXE".  Removed mention of "bin/" in
binary distributions, and "SCHEME_LARGE" and "SCHEME_COMPILER"
variables in build scripts; these are no longer used.  Replaced
"system" with "program" or "build tree" or whatnot, reserving "system"
for the thing that runs the "operating system".  Punted sections
"Building from source... ```The Hard Way''" and "... Tabula Rasa''",
which were mostly about re-compiling / re-linking the microcode.  The
solution to such problems is now automated by the compile-microcode
step in the Unix installation instructions.