Be more wary about 32-bit integer overflow in pg_stat_statements.

We've heard a couple of reports of people having trouble with
multi-gigabyte-sized query-texts files.  It occurred to me that on
32-bit platforms, there could be an issue with integer overflow
of calculations associated with the total query text size.
Address that with several changes:

1. Limit pg_stat_statements.max to INT_MAX / 2 not INT_MAX.
The hashtable code will bound it to that anyway unless "long"
is 64 bits.  We still need overflow guards on its use, but
this helps.

2. Add a check to prevent extending the query-texts file to
more than MaxAllocHugeSize.  If it got that big, qtext_load_file
would certainly fail, so there's not much point in allowing it.
Without this, we'd need to consider whether extent, query_offset,
and related variables shouldn't be off_t not size_t.

3. Adjust the comparisons in need_gc_qtexts() to be done in 64-bit
arithmetic on all platforms.  It appears possible that under duress
those multiplications could overflow 32 bits, yielding a false
conclusion that we need to garbage-collect the texts file, which
could lead to repeatedly garbage-collecting after every hash table
insertion.

Per report from Bruno da Silva.  I'm not convinced that these
issues fully explain his problem; there may be some other bug that's
contributing to the query-texts file becoming so large in the first
place.  But it did get that big, so #2 is a reasonable defense,
and #3 could explain the reported performance difficulties.

(See also commit 8bbe4cbd9, which addressed some related bugs.
The second Discussion: link is the thread that led up to that.)

This issue is old, and is primarily a problem for old platforms,
so back-patch.

Discussion: https://postgr.es/m/CAB+Nuk93fL1Q9eLOCotvLP07g7RAv4vbdrkm0cVQohDVMpAb9A@mail.gmail.com
Discussion: https://postgr.es/m/5601D354.5000703@BlueTreble.com

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