Declare read-only data arrays const, to warn on accidental modification
and let compilers place them in read-only memory sections.

Avoid the duplicate definition of strtoupper() in websocket.c, just use
the one from util.h.

The functions classifying characters like isspace(3) require the input
to be of type unsigned char or EOF.

When using just one job for processing the input lines do not use the
out-of-bounds index 1 instead of the valid one 0.  This currently seems
to only work since the stack space of the in this case unused threads
array is used.

Avoid array access in case the status code is less than 0 or bigger than
599.

Also reduce the size of the codes array.

Do not duplicate the strings for enumeration keys, as they are read-
only.

These two locations are only enabled via `--enable-debug`.