In cpstripped() and cptrimmed(), if a multibyte character is

found, only count it as taking up one character in the
destination buffer (if it has enought room).  This way,
scan(1)'s output won't be jagged if there are any subjects,
for example, that have multibyte characters.

Added a test for this to test-scan-multibyte, and moved the
test for an invalid multibyte sequence from test-scan to it.