Different set of conditions is used to verify
the validity of index definitions over a GEOMETRY
column in ALTER TABLE and CREATE TABLE. 
The difference was on how sub-keys notion validity
is checked.
Fixed by extending the CREATE TABLE condition to
support the cases allowed in ALTER TABLE.
Made the SHOW CREATE TABLE not to display spatial
indexes using the sub-key notion.


mysql-test/r/alter_table.result:
  Bug #26794: test case
mysql-test/r/gis-rtree.result:
  Bug #26794: fixed SHOW CREATE TABLE output.
mysql-test/t/alter_table.test:
  Bug #26794: test case
sql/field.cc:
  Bug #26794: Allow sub-keys for GEOMETRY
sql/sql_show.cc:
  Bug #26794: Don't show sub-key notion 
   in SHOW CREATE TABLE for SPATIAL indexes.
sql/sql_table.cc:
  Bug #26794: Allow sub-keys for GEOMETRY