do_count(): Improve debug printing
do_count(): Improve debug printing

1. Add a label to clearly show which of the recursive do_count() calls
   is done.
2. When do_count() returns a memoized value, show it immediately as
   M=value instead of a "return:" line. This reduces the volume of the
   debug output.
3. Use #define for the verbosity level.

Debug output sample (note the added l_* and r_* labels and M=):
l_any                do_count:500 lw=2 rw=11 le=(nil) re=Ds**x null_count=0
r_bnl                  do_count:480 lw=2 rw=4 le=(nil) re=(nil) null_count=0
                       return:480=0
r_any                  do_count:523 lw=4 rw=11 le=(nil) re=AN null_count=0
r_bnl                    do_count:480 lw=4 rw=5 le=(nil) re=(nil) null_count=0
                         return:480=1
r_any                    do_count:523 lw=5 rw=11 le=(nil) re=(nil) null_count=0
                         return:523=0
r_cmulti                 do_count:526 lw=5 rw=11 le=(nil) re=AN null_count=0
                         return:526=0
                       return:523=0
                     return:500=0
l_bnr                do_count(M=0):475 lw=11 rw=15 le=(nil) re=(nil) null_count=0
l_any                do_count:500 lw=2 rw=11 le=(nil) re=Ds**v null_count=0
r_bnl                  do_count(M=0):480 lw=2 rw=4 le=(nil) re=(nil) null_count=0

linas/link-grammar,opencog/link-grammar,ampli/link-grammar,ampli/link-grammar,opencog/link-grammar,linas/link-grammar,linas/link-grammar,linas/link-grammar,linas/link-grammar,opencog/link-grammar,opencog/link-grammar,linas/link-grammar,ampli/link-grammar,ampli/link-grammar,ampli/link-grammar,linas/link-grammar,ampli/link-grammar,ampli/link-grammar,ampli/link-grammar,opencog/link-grammar,opencog/link-grammar,opencog/link-grammar,ampli/link-grammar,linas/link-grammar,linas/link-grammar,opencog/link-grammar,opencog/link-grammar