Decodes modified UTF-7
text into 8bit data.
Returns the decoded 8bit data, or
FALSE if the input string was not valid modified
UTF-7. This function is needed to decode mailbox names that
contain international characters outside of the printable
ASCII range. The modified UTF-7 encoding is defined in
RFC 2060, section 5.1.3 (original UTF-7 was defned in
RFC1642).