After replacing sys_kcmp with syscall() and sys_prctl with prctl(), the
API is changed to return -1 and set errno on failure (instead of
returning the negative value of the error code directly on return.)

Commit 8ceab588 ("crtools: no more linked with builtin syscall")
replaced calls to sys_kcmp and sys_prctl, but did not update the checks
for ret to check for -1 and errno, so update them here.

Also make the comparisons for the return value explicit checks for a
negative value (expected -1) instead of just failing on a non-zero value
with the implicit comparison.

Add %m to report what errno was set to on failed syscalls, to make it
easier to troubleshoot a check failure.

Tested that now `criu check --ms` stopped reporting the misleading:
  prctl: One needs CAP_SYS_RESOURCE capability to perform testing.
Instead, now it reports:
  Error (cr-check.c:165): System call kcmp is not supported: No such process
  Warn  (cr-check.c:195): Skipping unssuported PR_SET_MM_MAP: Invalid argument
  prctl: PR_SET_MM is not supported: Invalid argument
And those messages include the error message that explains why the
syscall failed.
Reported-by: Saied Kazemi <saied@google.com>
Fixes: 8ceab588 ("crtools: no more linked with builtin syscall")
Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>