Problem: ":sign place" only uses the current buffer.
Solution: List signs for all buffers when there is no buffer argument.
Fix error message for invalid buffer name in sign_place().
(Yegappan Lakshmanan, closes#3774)
Problem: Sign message not translated and inconsistent spacing.
Solution: Add _() for translation. Add a space. (Ken Takata) Also use
MSG_BUF_LEN instead of BUFSIZ.
Problem: ":sign place" requires the buffer argument.
Solution: Make the argument optional. Also update the help and clean up the
sign test. (Yegappan Lakshmanan, closes#3767)
Problem: Functionality for signs is spread out over several files.
Solution: Move most of the sign functionality into sign.c. (Yegappan
Lakshmanan, closes#3751)
