Invent system_functions.sql to carry the function definitions that
were formerly in system_views.sql.  The function definitions were
already a quarter of the file and are about to be more, so it seems
appropriate to give them their own home.

In passing, fix an oversight in dfb75e47: it neglected to call
check_input() for system_constraints.sql.

Discussion: https://postgr.es/m/3956760.1618529139@sss.pgh.pa.us