gen-strtab.awk: Fix strings that start with octal digits.

gen-strtab.awk: Fix strings that start with octal digits.

The switch to a single string literal broke table when a string (other
than the first in the table) starts with an octal digit.  Since the
prior string ends with \0, such a digit will get eaten by the octal
escape and mess everything up.

Since an octal escape has at most 3 digits, the solution is simply to
use \000 instead of \0.  To improve readability of the output, check
if it is actually needed and emit \0 or \000 as appropriate.