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 dfb75e478: it neglected to call
check_input() for system_constraints.sql.
Discussion: https://postgr.es/m/3956760.1618529139@sss.pgh.pa.us
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