`readhuditem` does not parse V_ constants

atoi is called for the part of the string after the assignment operator, which is sufficient for x and y coordinates but not for flag constants. This makes it impossible to adjust HUD item flags in SOC without using magic numbers. https://git.do.srb2.org/STJr/SRB2/-/blob/next/src/deh_soc.c#L2678

There's also a warning at the bottom of the function that erroneously describes the HUD item as a level header.