Move extern declarations for EXEC_BACKEND to header files

Move extern declarations for EXEC_BACKEND to header files

This fixes warnings from -Wmissing-variable-declarations (not yet part
of the standard warning options) under EXEC_BACKEND.  The
NON_EXEC_STATIC variables need a suitable declaration in a header file
under EXEC_BACKEND.

Also fix the inconsistent application of the volatile qualifier for
PMSignalState, which was revealed by this change.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/e0a62134-83da-4ba4-8cdb-ceb0111c95ce@eisentraut.org