CS0 is not working as input by default

inputs seems to have pull-ups

on the popular CH341A-based programmer "RUN" LED is connected to SDA and CS0, active low

Software (bit-banged) I2C (CH341SoftwareI2C, used e.g. by PN532 over I2C)
requires RXD connected to SCL:
- CH341 SCL is output-only (CH341SetOutput bit 18) and cannot be read back;
  CH341GetInput has no SCL bit (SDA is readable as bit 23).
- Clock stretching (slave holding SCL low while busy, e.g. PN532) needs the
  real SCL state, so SCL is wired to a readable pin: input bit 9 = PEMP pin,
  which should be the pin labeled RXD on the programmer header (ERR# = TXD,
  PEMP = RXD in UART mode - not verified against the datasheet).
- Both CH341 SCL and RXD go to the same slave SCL line - it is one net:

    CH341 SCL --+-- slave SCL
    CH341 RXD --+     (RXD only reads, never drives)
    CH341 SDA ------- slave SDA

- SCL/SDA need pull-ups (typ. 4.7k to 3.3 V; most breakout boards have them).
  CH341 SCL/SDA are open-drain, so with pull-ups to 3.3 V a 5 V CH341A board
  does not push 5 V onto the bus.
- Without the RXD-SCL jumper CH341SoftwareI2C::begin() fails with
  "SCL input (RXD) reads low ... is RXD connected to SCL?" in the log.
