Improve standards conformance of string->number.
Fixes <http://bugs.gnu.org/11887>.
* libguile/numbers.c (mem2ureal): New argument 'allow_inf_or_nan'.
Accept infinities and NaNs only if 'allow_inf_or_nan' is true and "#e"
is not present. Check for "inf.0" or "nan." case-insensitively. Do
not accept rationals with zero divisors.
(mem2complex): Pass new 'allow_inf_or_nan' argument to 'mem2ureal',
which is set if and only if a explicit sign was present.
* test-suite/tests/numbers.test ("string->number"): Add tests.