strtonum slight improvement
github-issue: #32

Based on PR 165970, some improvements are made to make it more straight.
1/ As the internal struct is not used anymore to store errno context,
no need to keep the couple { NULL, 0 } anymore and it is just readable
variable, we can make it const.
2/ In first place, checking EINVAL is not necessary (and not really
reliable), its test is removed checking ep pointer is enough.
3/ Checking ERANGE in first place for out of ranges tests, LLONG_MIN/MAX
values could be legitimate.
4/ If all is good, it returns everything right away with the saved errno
context.