Check for illegal character '=' in environment variable names.

Check for illegal character '=' in environment variable names.

Make sure the string passed as variable name does not contain a '='
character. This not only prevents the common error or typing
"setenv foo=bar" instead of "setenv foo bar", but (more importantly)
also closes a backdoor which allowed to delete write-protected
environment variables, for example by using "setenv ethaddr=".