Some ports (like `bc` with history enabled) sensibly set the termios
character size to 8 bits.
Previously, we left the character size value (given by the bitmask
CSIZE) as zero by default (meaning 5 bits per character), and returned
ENOTIMPL whenever someone modified it. This was dumb.
This commit adds support for the various ECHO* lflags and fixes some
POSIX conformance issues around newline handling. Also included are
error messages when setting not implemented settings.
This non-POSIX header is used in Linux/BSD systems for storing the
default termios settings. This lets us setup new TTYs' `m_termios.c_cc`
in a nicer way than using a magic string.
