Rename fkey.[h,c] to fk_constraint.[hc]

src/box/CMakeLists.txt

src/box/alter.cc

src/box/fk_constraint.c

src/box/fk_constraint.h

src/box/sql.c

src/box/sql/CMakeLists.txt

src/box/sql/build.c

src/box/sql/fk_constraint.c

src/box/sql/pragma.c

src/box/sql/vdbe.c