Implement grok_atou as safe/strict atoi replacement.

For earlier discussion, see:
http://www.nntp.perl.org/group/perl.perl5.porters/2013/10/msg208680.html
https://rt.perl.org/Public/Bug/Display.html?id=116118#txn-1250187

grok_atou is completely new code, instead of trying to bolt
new parameters/flags to grok_number.  This makes it easier to
be extremely strict, and not worry about breaking grok_number.
Use grok_atou instead of atoi.

Remaining atoi() uses include at least:
ext/DynaLoader/dl_aix.xs, os2/os2.c, vms/vms.c
Add tests for grok_atou.
Add atoi to avoidables.
Advertise grok_atou.
Use grok_atou instead of strtoul (no explicit strtol uses).
Document grok_atou as strtoul replacement.
Add strtoul and strtol to avoidables.
Atol can be strtol in disguise, so grok_atou.
Introduce maxend for paranoia.  Also, comments.