Clean up warnings

[mthowe]

dvtm.c: In function ‘drawbar’:
dvtm.c:380:4: warning: implicit declaration of function ‘addnwstr’; did you mean ‘addnstr’? [-Wimplicit-function-declaration]
addnwstr(wbuf+i, 1);
^~~~~~~~
addnstr
dvtm.c: In function ‘parse_args’:
dvtm.c:1844:10: warning: passing argument 1 of ‘free’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
free(fifo);
^~~~
In file included from dvtm.c:13:0:
/usr/include/stdlib.h:563:13: note: expected ‘void *’ but argument is of type ‘const char *’
extern void free (void *__ptr) __THROW;
^~~~
At top level:
dvtm.c:1262:1: warning: ‘focusprevnm’ defined but not used [-Wunused-function]
focusprevnm(const char *args[]) {
^~~~~~~~~~~
vt.c:518:13: warning: ‘buffer_row_prev’ defined but not used [-Wunused-function]
static Row *buffer_row_prev(Buffer *b, Row *row)
^~~~~~~~~~~~~~~
vt.c:487:13: warning: ‘buffer_row_last’ defined but not used [-Wunused-function]
static Row *buffer_row_last(Buffer *b) {