Various parts of code used different 'precision' arguments for sprintf("%g") when converting
floating point numbers to a string. This led to differences in results in some cases 
depending on whether the text-based or prepared statements protocol is used for a query.

Fixed by changing arguments to sprintf("%g") to always be 15 (DBL_DIG) so that results are
consistent regardless of the protocol.

This patch will be null-merged to 6.0 as the problem does not exists there (fixed by the
patch for WL#2934).

client/sql_string.cc:
  Use 15 (DBL_DIG) as a precision argument for sprintf(), as Field_double::val_str() does.
libmysql/libmysql.c:
  Use 15 (DBL_DIG) as a precision argument for sprintf(), as Field_double::val_str() does.
mysql-test/r/archive_gis.result:
  Fixed test results to take additional precision into account.
mysql-test/r/func_group.result:
  Fixed test results to take additional precision into account.
mysql-test/r/func_math.result:
  Fixed test results to take additional precision into account.
mysql-test/r/func_str.result:
  Fixed test results to take additional precision into account.
mysql-test/r/gis.result:
  Fixed test results to take additional precision into account.
mysql-test/r/innodb_gis.result:
  Fixed test results to take additional precision into account.
mysql-test/r/select.result:
  Fixed test results to take additional precision into account.
mysql-test/r/sp.result:
  Fixed test results to take additional precision into account.
mysql-test/r/type_float.result:
  Fixed test results to take additional precision into account.
mysql-test/t/type_float.test:
  Fixed test results to take additional precision into account.
sql/sql_string.cc:
  Use 15 (DBL_DIG) as a precision argument for sprintf(), as Field_double::val_str() does.