[RDY] Re-integrate FEAT_SIGNS code #383

- restore FEAT_SIGNS and related signs* code from Vim origin
- omit FEAT_NETBEANS_INT, FEAT_SIGN_ICONS, FEAT_GUI code
- ex_cmds.c and buffer.c are probably good candidates for moving to a new signs.c file, in another PR perhaps
- deferring style guidelines to a separate PR