Merge r1611858, r1611908, r1611916 from trunk

doc: improve doxygen output for util_ebcdic.h and util_charset.h.

Submitted by: jailletc36
Reviewed by: jailletc36, rjung, ylavic
Backported by: jailletc36

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1635905 13f79535-47bb-0310-9956-ffa450edef68