[RFC] Extend namespace of valid guc names

Hi,

Currently guc-file.c allows the following as guc names:

ID        {LETTER}{LETTER_OR_DIGIT}*
QUALIFIED_ID    {ID}"."{ID}

That is either one token starting with a letter followed by numbers or
letters or exactly two of those separated by a dot.
Those restrictions are existing for neither SET/set_config() via SQL nor
for postgres -c styles of setting options.

I propose loosening those restrictions to
a) allow repeatedly qualified names like a.b.c
b) allow variables to start with a digit from the second level onwards.

Additionally, should we perhaps enforce the same rules for -c and
set_config()/SET?

Trivial patch that only extends the space of valid names for
postgresql.conf attached.

Greetings,

Andres Freund

--
 Andres Freund                       http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services