Add clock support to MOS

[breakintoprogram]

Acceptance criteria:

• The ability to set the time with a star command (maybe *TIME, or split into *DATE and *TIME)
• The ability to read the time with a star command (maybe the same command, no parameters)
• Time can be set and read using MOS api calls
• Time can be used in FatFS as a timestamp

Nice to have:

• Check whether the EZ80F92 has a RTC (yes, but not fully plumbed in - see comments below)
• Check whether the ESP32 has a RTC (yes)

[breakintoprogram]

Tested with Agon Light Issue 1 and it looks like the RTC requires an external 32768hz crystal connected to its oscillator. Investigating using a timer to provide a software option. Future proof to make it easy to plug in a RTC module and / or use the eZ80 built in one (for future revisions).

[breakintoprogram]

Using ESP32 RTC is working out okay:

• VDU 23, 0, 7. 0 to request the time
• VDU 23, 0, 7, 1, Y - 1980, M, D, H, M, S to set the time
• This is sent back as a packet from the ESP32 to the vdp protocol handler

Changes:

• Added *TIME command
• Added mos_getrtc
• Added mos_setrtc

NB:

• Time does not survive reset
• Clock doesn't handle dates before 1970