Restore the contract on all convert_stringXX() interfaces. Add a "allow_bad_conv"

Restore the contract on all convert_stringXX() interfaces. Add a "allow_bad_conv"
boolean parameter that allows broken iconv conversions to work. Gets rid of the
nasty errno checks in mangle_hash2 and check_path_syntax and allows correct
return code checking.
Jeremy.
(This used to be commit 7b96765c23637613f079d37566d95d5edd511f05)