ComRx_GetHex [D>=23]

Return a hex number from the head of the Rx buffer. Skip any leading spaces. Terminate thereafter after 2 hex digits or on first non-hex character. Accept upper or lower case. Consume the characters used but not the terminating character. Result in X, 0 if nothing there. R register signals outcome. 0 = OK, 1 = no valid hex number present.

Hex numbers longer than 8 bits can be read in several grabs.

Dialect exclusions: Not available in dialects before 23