ALTER SYSTEM itself normally won't make duplicate entries (although
up till this patch, it was possible to confuse it by writing case
variants of a GUC's name).  However, if some external tool has appended
entries to the file, that could result in duplicate entries for a single
GUC name.  In such a situation, ALTER SYSTEM did exactly the wrong thing,
because it replaced or removed only the first matching entry, leaving
the later one(s) still there and hence still determining the active value.

This patch fixes that by making ALTER SYSTEM sweep through the file and
remove all matching entries, then (if not ALTER SYSTEM RESET) append the
new setting to the end.  This means entries will be in order of last
setting rather than first setting, but that shouldn't hurt anything.

Also, make the comparisons case-insensitive so that the right things
happen if you do, say, ALTER SYSTEM SET "TimeZone" = 'whatever'.

This has been broken since ALTER SYSTEM was invented, so back-patch
to all supported branches.

Ian Barwick, with minor mods by me

Discussion: https://postgr.es/m/aed6cc9f-98f3-2693-ac81-52bb0052307e@2ndquadrant.com