uart: rework text formatting of communicated data values, plus nits

uart: rework text formatting of communicated data values, plus nits

Factor out the code which generates a textual representation for the
numeric values that were communicated via UART bit patterns. Make the
width of the output text depend on the number of bits in the UART frame
(five to nine) instead of assuming bytes of exactly eight bits.

Fix other minor issues while we are here: Nine bits result in a number
range of 0 to 511 (not 512). ASCII codes 30 and 31 are non-printables.
The previous implementation skipped a significant leading digit in the
octal representation.

Signed-off-by: Gerhard Sittig <redacted>