Add atoid() function to convert a string to an id_t (uid, gid or

Add atoid() function to convert a string to an id_t (uid, gid or
pid).  We have to be careful to choose() either strtol() or strtoul()
depending on whether the string appears to be signed or unsigned.
Always using strtoul() is unsafe on 64-bit platforms since the
uid might be represented as a negative number and (unsigned long)-1
on a 64-bit system is 0xffffffffffffffff not 0xffffffff.
Fixes a problem with uids larger than 0x7fffffff on 32-bit platforms.