Add LOCK A,B,C functionality as LOCK A;LOCK B;LOCK C; as agreed.

doc/src/sgml/ref/lock.sgml

src/backend/commands/command.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/parser/gram.y

src/include/nodes/parsenodes.h

src/interfaces/ecpg/preproc/preproc.y