libbb: use ptsname_r, it's smaller

function                                             old     new   delta
xgetpty                                               91      81     -10
ptsname                                               33       -     -33

   text    data     bss     dec     hex filename
 793828     592    6692  801112   c3958 busybox_old
 793796     592    6660  801048   c3918 busybox_unstripped

1 file changed