получить настраиваемые системные переменные (get configurable system variables)
Возвращаемое значение (Return value)
If name is an invalid value, sysconf() shall return -1 and set
errno to indicate the error. If the variable corresponding to
name is described in <limits.h> as a maximum or minimum value and
the variable has no limit, sysconf() shall return -1 without
changing the value of errno. Note that indefinite limits do not
imply infinite limits; see <limits.h>.
Otherwise, sysconf() shall return the current variable value on
the system. The value returned shall not be more restrictive than
the corresponding value described to the application when it was
compiled with the implementation's <limits.h> or <unistd.h>. The
value shall not change during the lifetime of the calling
process, except that sysconf(_SC_OPEN_MAX) may return different
values before and after a call to setrlimit() which changes the
RLIMIT_NOFILE soft limit.
If the variable corresponding to name is dependent on an
unsupported option, the results are unspecified.
