pgsql: Split function definitions out of system_views.sql into a new fi

Split function definitions out of system_views.sql into a new file.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e80949372564c126c92aa7d64de483e04c0ef95e

Modified Files
--------------
src/backend/catalog/Makefile | 3 +-
src/backend/catalog/system_functions.sql | 392 +++++++++++++++++++++++++++++++
src/backend/catalog/system_views.sql | 381 +-----------------------------
src/bin/initdb/initdb.c | 6 +
4 files changed, 401 insertions(+), 381 deletions(-)