Fixed a typo in a code comment for LM-Response
A number of fixes (mainly comments) of rlm_expr

* Update the urlunquote example to make it copy-pastable by doubling the %. Add a comment why
* Typo fix: s/wont'/won't/
* Removed UNUSED macro for *instance in (un)escape_xlat, it is being used here
* Fix the unuescape example, the name here should be unescape, not escape
* Added a ":" after "Example" in tolower. This makes it easier to find the examples in this file, since all other examples did this.
Define sig_t for systems without it.  Closes #765
Merge pull request #763 from qnet-herwin/documentation

Updates to code comments
Trigger max_threads upon reaching the limit

Trigger "max_threads" event (and thus "threadMaxThreads" trap, with
default SNMP configuration) upon reaching "max_servers" thread limit,
instead of on an attempt to exceed it, as the latter never happens.
Add debugging policies
Add credits for comp128 v2/v3 code
Add event timestamp when preprocess is called in authorize
Add missed & references in default policies
No where else calls the string representation of VALUE_PAIR a pairstr
Missing declaration