libc: add getgrnam_r for porting CRuby
libc: move getgrnam from runtime.cc
libc: add getpwnam_r to porting 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
libc: add setreuid for porting CRuby
libc: add setregid for porting CRuby
libc: add waitpid for porting CRuby
setting option for build and debug
libc: add pthread_attr_getguardsize() for porting CRuby
mempool: add malloc_usable_size() to C linkage for porting CRuby
README: add description about building for Arch user