Have psql escape bytes in strings for variables follow the backend

Have psql escape bytes in strings for variables follow the backend
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.