Fix all compile-time warnings

[kfogel]

Before this change:

  $ make
  gcc -c -g -O3 -Wunused-variable -DFONTDIR='"/usr/local/share/dcled"' \
                                  -DDCLEDVERSION='"2.2"' \
                                  -I/usr/include/libusb-1.0 dcled.c
  dcled.c: In function ‘open_usbdev’:
  dcled.c:265:3: warning: ‘libusb_set_debug’ is deprecated: \
                 Use libusb_set_option instead [-Wdeprecated-declarations]
     libusb_set_debug(NULL,3);
     ^~~~~~~~~~~~~~~~
  In file included from dcled.c:30:
  /usr/include/libusb-1.0/libusb.h:1300:18: note: declared here
   void LIBUSB_CALL libusb_set_debug(libusb_context *ctx, int level);
                    ^~~~~~~~~~~~~~~~
  dcled.c: In function ‘printBCDchar’:
  dcled.c:409:7: warning: implicit declaration of function ‘isdigit’ \
                 [-Wimplicit-function-declaration]
    if (!isdigit(c)) {
         ^~~~~~~
  dcled.c: In function ‘savefonts’:
  dcled.c:1375:30: warning: implicit declaration of function ‘isgraph’ \
                   [-Wimplicit-function-declaration]
      fprintf(out,"%02x %c ",i,(isgraph(i)?i:'.'));
                                ^~~~~~~
  gcc dcled.o -o dcled -g -lm -lusb-1.0
  gcc -c -g -O3 -Wunused-variable -DFONTDIR='"/usr/local/share/dcled"' \
                                  -DDCLEDVERSION='"2.2"' \
                                  -I/usr/include/libusb-1.0 cpuload.c
  cpuload.c: In function ‘main’:
  cpuload.c:85:7: warning: implicit declaration of function ‘strcmp’ \
                  [-Wimplicit-function-declaration]
     if(!strcmp(tag,"cpu ")) {
         ^~~~~~
  gcc cpuload.o -o cpuload -g -lm -lusb-1.0
  $

After this change:

  $ make
  gcc -c -g -O3 -Wunused-variable -DFONTDIR='"/usr/local/share/dcled"' \
                -DDCLEDVERSION='"2.2"' -I/usr/include/libusb-1.0 dcled.c
  gcc dcled.o -o dcled -g -lm -lusb-1.0
  gcc -c -g -O3 -Wunused-variable -DFONTDIR='"/usr/local/share/dcled"' \
                -DDCLEDVERSION='"2.2"' -I/usr/include/libusb-1.0 cpuload.c
  gcc cpuload.o -o cpuload -g -lm -lusb-1.0
  $

[kfogel]

I'm probably not going to merge this pull request into the Conservatory copy of the code. At the same time, though I'm not prepared to take over maintenance of dcled. So I'm just making the PR visible for now, in case anyone else runs into these compile-time warnings. If I hear from other folks here, that'll be a sign that maybe I should actively maintain @kfogel/dcled after all.