Use strlen even for constant strings

GCC optimizes strlen("string constant") to a constant, even with -O0.
Thus, replace patterns like sizeof("string constant")-1 with
strlen("string constant") where possible, for clarity.  In particular,
for expressions intended to add up the lengths of components going into
a string, this often makes it clearer that the expression counts the
trailing '\0' exactly once, by putting the +1 for the '\0' at the end of
the expression, rather than hidden in a sizeof in the middle of the
expression.