Add allow_alter_system GUC.

This is marked PGC_SIGHUP, so it can only be set in a configuration
file, not anywhere else; and it is also marked GUC_DISALLOW_IN_AUTO_FILE,
so it can't be set using ALTER SYSTEM. When set to false, the
ALTER SYSTEM command is disallowed.

There was considerable concern that this would be misinterpreted as
a security feature, which it is not, because a determined superuser
has various ways of bypassing it. Hence, a lot of work has gone into
wordsmithing the documentation, in the hopes of avoiding any such
confusion.

Jelte Fennemia-Nio and Gabriele Bartolini, with wording suggestions
for the documentation from many others.

Discussion: http://postgr.es/m/CA%2BVUV5rEKt2%2BCdC_KUaPoihMu%2Bi5ChT4WVNTr4CD5-xXZUfuQw%40mail.gmail.com

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