Implement Base64URL-encoded SHA512 checksums

Implement Base64URL-encoded SHA512 checksums

	* ini.h: Add macros for use within the implementation of the
	checksum parsers.  Hexdigest requires a 2-to-1 and Base64 a 4-to-3
	conversion.  Base64 uses the filename and URL safe alphabet from
	RFC4648.  It would be trivial to additionally process the normal
	Base64 alphabet but we don't want to allow that in order to be
	able to use the checksums as filenames without further conversion.
	(hexnibble, b64url): Process single input character to input value.
	(nibbled1): Convert 2 processed input values into 1 output value.
	(b64d1, b64d2, b64d3): Convert 4 input values into 3 output
	values.
	* iniparse.yy: Add SHA512B64URL checksum in addition to MD5 and
	SHA512.  Keep symmetry with MD5LINE and also define SHA512LINE
	syntax.
	* inilex.ll: Implement existing MD5 and SHA512 checksum parsers
	using new macros.  Implement new SHA512B64URL parser using new
	macros.  Enforce Base64 alphabet as defined in RFC4648 with no
	padding to enable direct use of checksum values as filenames.
	Implement SHA512LINE parser.