We only need to transmit the ones where we differ from rfc7540's initial
values.

Of a VSM segment, we map the least number of pages from the offset into the file
covering the segment length, aligned to page boundaries. For calculating
relative pointers into the mapping and the length, we need to deduce the initial
page boundary offset.

Fixes #2541

Some specialcasing is warranted to get sensible messages.

Fixes #2532

stdin-fd is dead, it takes more than "break;" to get out of a
switch-in-a-loop.

Also: Make ansi-sequence problems fatal, and render unknown control
chars as '?'

I owe the next round Geoff.