use str* API in place of isdigit() in server/conf.c

[clepple]

isdigit() takes a char/int, not a char*.

Originally mentioned here, but we need to fix this before the next release.

[clepple]

I'm bumping the milestone from 2.7.5 to 2.7.6 because we merged #690, which fixes the misuse of isdigit(). I still think that a slightly reworked version of #677 (with signed/unsigned expanded out into separate variables, etc.) should be merged into the following release.