lib/, src/: Fix error handling after strto[u]l[l](3)

lib/, src/: Fix error handling after strto[u]l[l](3)

-  Set errno = 0 before the call.  Otherwise, it may contain anything.
-  ERANGE is not the only possible errno value of these functions.  They
   can also set it to EINVAL.
-  Any errno value after these calls is bad; just compare against 0.
-  Don't check for the return value; just errno.  This function is
   guaranteed to not modify errno on success (POSIX).
-  Check endptr == str, which may or may not set EINVAL.

Suggested-by: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>