Print floats readably when fpprintprec = 0 or fpprintprec > 16.

Print floats readably when fpprintprec = 0 or fpprintprec > 16.

 * Update Texinfo documentation about fpprintprec.

 * src/commac.lisp:

   in EXPLODEN-FORMAT-FLOAT-READABLY, call PRIN1-TO-STRING to ensure readable output

   in EXPLODEN-FORMAT-FLOAT-PRETTY, simplify handling of fpprintprec in FORMAT arguments

 * src/testsuite.lisp:

   rtest6: cut out references to old ECL versions, and cut out
   expected failure for test 46, as there are only 45 tests at present

   rtest16: renumber expected failures

 * tests/rtest6.mac:

   allow additional variation in displayed exponents since FORMAT "~e"
   always outputs a sign on the exponent, and PRIN1-TO-STRING might or might not

   additional test cases for parse_string(string(foo)) in the presence
   of fpprintprec = 0, for foo = least_positive_float and friends

 * tests/rtest16.mac: separate tests for fpprintprec into cases 0 or > 16 versus 2 to 16,
   and output some additional detail for failed tests

Fixes SF bug #4107: "least_positive_float doesn't print/read correctly in float/bfloat"