s3:lib/util_str: add strlen_m_ext() that takes input and output charset

s3:lib/util_str: add strlen_m_ext() that takes input and output charset

The function calculates the number of units (8 or 16-bit, depending
on the destination charset), that would be needed to convert the
input string which is expected to be in in src_charset encoding
to the dst_charset (which should be a unicode charset).