saslAuthzTo/From stuff

when comparing IDs to saslAuthzTo/From values, the saslAuthzTo
saslAuthzFrom values can take different forms:

dn[.<style>]:<pattern>

<style> ::= 	exact		; exact match
		children	; children of <pattern> match
		subtree		; <pattern> or children of <pattern> match
		regex		; <pattern> is regcomp() & regexec()
if no <style>, then exact is assumed

u[.<mech>][/<realm>]:<user>

when parsing a proxyAuthz value, only exact DN is allowed,
and no <mech> can be specified.  <user> cannot contain ':'
and <mech> cannot contain '/'.