Exit error codes improvements, some formatting

According to programming standards e.g. C-style defines) testssl.sh has now
internal error variables (ERR_*) which are defined to deal with exit codes in
error conditions. Details see ``testssl.sh(1)``, section exit.  Thus exit codes
because of an error are now standardized and if needed can be easily changed to
other values.

This is part of a cleanup mentioned in #985 and #752. Codes for monitoring
tools (#327) which imply some kind of rating are still to be done.

The beginning section was reformatted and some items were reordered to keep
variables and functions together which serve similar purposes.

``readonly`` was replaced by ``declare -r`` (closer to C's define and it
makes more sense to settle on one variable if both are being used
for the same purpose)

Author: drwetter