strbuf: reduce default buffer size

Using 2048 as buffer sizer for strbuf is a bit exaggerated. strbuf is
used much more when we are not using mmapped indexes, but it's used for
mmapped when for example searching for an alias. A quick and dirty hack
to output the size of our strbufs is to print buf->used inside
strbuf_str(). Doing this and creating some statistics with:

while read xxx alias xxx; do
    tools/modprobe -R "$alias" > /dev/null;
done < /lib/modules/$(uname -r)/modules.alias 2>&1 | \
	     Rscript -e 'summary (as.numeric (readLines ("stdin")))'

   Min. 1st Qu.  Median    Mean 3rd Qu.    Max.    NA's
   1.00   29.00   31.00   31.55   38.00   92.00      26

So, reduce the step to 128, which is still greater than the maximum in
these cases. In the worst case this can only create a few calls to
realloc(), while keeping the memory footprint low for the common cases.
strbuf: do not calculate next step in size on all calls

We only need to check if the new size is less or equal than the current
size. We don't really need to calculate the next step.
strbuf: make strbuf_pushchars() a little less dumb

Do not push one char at a time. We have strlen() and memcpy() to rely
on.