ddb: enable the use of ^C and ^S/^Q

This lets one interrupt DDB's output, which is useful if paging is
disabled and the output device is slow.

This follows a previous implementation in svn r311952 / git
5fddef79998678d256ba30316353393b4d8ebb32 which was reverted because it
broke DDB type-ahead.

Now, try this again, but with a 512-byte type-ahead buffer. While there
is buffer space, control input is handled and non-control input is
buffered. When the buffer is exhausted, the default is to print a
warning and drop further non-control input in order to continue handling
control input. sysctl debug.ddb.prioritize_control_input can be set to
0 to instead preserve all input but lose immediate handling of control
input. This could for example effect pasting of a large script into the
ddb console.

Suggested by: Anton Rang <rang@acm.org>
Reviewed by: markj
Discussed with: imp
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D28676