conventions of only allowing octal, like \045.  Remove support for
\decimal, \0octal, and \0xhex which matches the strtol() function but
didn't make sense with backslashes.

These now return the same character:

	test=> \set x '\54'
	test=> \echo :x
	,
	test=> \set x '\054'
	test=> \echo :x
	,

THIS IS A BACKWARD COMPATIBILITY CHANGE.