Prevent error recursion crashes when encoding conversion fails (Tom)

This change extends fixes made in the last two minor releases for related failure scenarios. The previous fixes were narrowly tailored for the original problem reports, but we have now recognized that any error thrown by an encoding conversion function could potentially lead to infinite recursion while trying to report the error. The solution therefore is to disable translation and encoding conversion and report the plain-ASCII form of any error message, if we find we have gotten into a recursive error reporting situation. (CVE-2009-0922)

Disallow CREATE CONVERSION with the wrong encodings for the specified conversion function (Heikki)

This prevents one possible scenario for encoding conversion failure. The previous change is a backstop to guard against other kinds of failures in the same area.

Fix core dump when to_char() is given format codes that are inappropriate for the type of the data argument (Tom)

Add MUST (Mauritius Island Summer Time) to the default list of known timezone abbreviations (Xavier Bugaud)