libc: add getgrnam_r for porting CRuby
libc: add getpwnam_r to portin CRuby

getpwnam_r and getpwuid_r is almost same function.
So, implement getpw_r for commonly function and
To change the two function to call getpw_r
libc: add function of pgid for porting CRuby
libc: add getpgrp for porting CRuby
libc: add getppid for porting CRuby
libc: add sid function for porting CRuby
libc: add setegid for porting CRuby
libc: add seteuid for porting CRuby