Begin fixing uses of strtol: parse integers more carefully.

Begin fixing uses of strtol: parse integers more carefully.
Patch from Jim Meyering
* src/internal.h: Include <errno.h>.
  Define new static inline function, xstrtol_i.
* src/virsh.c: Detect integer overflow in domain ID number
  in vshCommandOptDomainBy. Detect overflow and invalid port
  number suffix in cmdVNCDisplay.
* src/xend_internal.c: Parse CPU number more carefully in
  xenDaemonDomainGetVcpus.
* tests/int-overflow: New script. Test for the above-fixed bug.
* tests/Makefile.am: Add int-overflow to TESTS. Define
  TESTS_ENVIRONMENT, to propagate $abs_top_* variables into the
  int-overflow script. Adapt the "valgrind" rule not to clobber
  new TESTS_ENVIRONMENT.
Daniel