More additions to unlang

[Alan DeKok]

  The changes in today add "compile-time" validation, and evaluation of
static values.  For example:

 	if (Session-Timeout < blue) {

  In v2, the server will start, and give run-time errors.

  In v3, the server will refuse to start, as "blue" is not a valid number.

  Even better, parts of the virtual servers can now be removed at start
time:

	if (0) {
		sql
		ldap
	}

  When the contents of a condition are statically equivalent to false
(e.g. 0), then the contents of the "if" block are completely ignored.
It lets you have conditional parts of the configuration, which have zero
run-time CPU or memory cost.

  Alan DeKok.