rework: login.c

[Walter Harms]

hi list,
i did some rework of the 13.0 login code and fixed some border cases.
mostly code cleanup to improve readability.
the important step is that getpwnam_r now uses
sysconf(_SC_GETPW_R_SIZE_MAX) to allocate enough space. My version of
glibc reports 1024 that is much more that the current limit of 256. maybe
that should be changed.

denys,
i made a lot of small changes please check if want to copy some changes
into the current code.

here is the PAM case

Final link with: pam_misc
function                                             old     new   delta
check_pswd                                             -     447    +447
run_login_script                                       -     209    +209
my_getpwnam_r                                          -     100    +100
login_main                                          1851    1043    -808
------------------------------------------------------------------------------
(add/remove: 3/0 grow/shrink: 0/1 up/down: 756/-808)          Total: -52
bytes
   text    data     bss     dec     hex filename
  30238     685    8284   39207    9927 busybox_old
  30240     689    8284   39213    992d busybox_unstripped
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