Introduce Util::parse_{unsigned,signed} functions

Introduce Util::parse_{unsigned,signed} functions

parse_unsigned replaces parse_uint32 while parse_signed replaces
parse_int. For simplicity, both return 64-bit values; I see no need to
be able to return narrower types since they are used for parsing
configuration values where a valid range is more important. Therefore
the functions optionally verify minimum and maximum allowed values.