make regex_old.c, ruserpass.c use __uc_malloc,

make regex_old.c, ruserpass.c use __uc_malloc,
replace "buf = malloc(BUFSIZ); if (!buf) abort();" by __uc_malloc
elsewhere.

With last 7 patches together uclibc has 3k of static data total
with fairly big .config and with 2k being used for 2 x BUFSIZ stdio buffer:
   text    data     bss     dec     hex filename
    114     132    2048    2294     8f6 _stdio.o (ex lib/libc.a)
total data 593
total bss  3062