[doc/mpfr.texi] Completed specification:

  * on special numbers (±0, ±Inf);
  * of mpfr_lngamma and mpfr_lgamma on 1 and 2 (result is +0);
  * general rules for special values and on mpfr_lngamma.
[src/lngamma.c] Fixed mpfr_lngamma behavior on nonpositive integers
  and -Inf.
[tests/tlngamma.c] Update. Test the flags for some special values.
See <https://sympa.inria.fr/sympa/arc/mpfr/2013-08/msg00008.html> about
the behavior of mpfr_lngamma on zero and general rule for such limits.
(merged changesets r8629,8632-8633,8841,8847 from the trunk)

[From SVN r9605 (branches/3.1)]