This commit adds validation of the values of the ssl-mode parameter
in SSL scripts, since now only a basic check for the presence of the
"VERIFY_" prefix is performed there to detect "VERIFY_IDENTITY" and
"VERIFY_CA", but all other values are not checked at all.

In addition, this commit removes leading and trailing spaces from
parameter values that SST scripts read from configuration files or
from the command line so that they do not interfere with parameter
checks and substitutions. Parameter substitution has been made more
robust against characters in strings that the shell might erroneously
interpret as regexp.