Make it easier to use strtoi and strtou in downsteam applications
without the need to define HAVE_NBTOOL_CONFIG_H and yet allow -Wundef
not to log any warnings.

- new test for strtoi
- namespace protection for strto{i,u}
- separate manpages for strto{i,u} from the ones for strto{u,}l
From: Kamil Rytarowski

add strto{i,u} from Kamil Rytarowski as discussed in tech-userlevel.