* sysdeps/unix/sysv/linux/syscalls.list: Add __syscall_setresuid
as real name for system call and make setresuid a weak alias. Set
caller to seteuid.
* sysdeps/unix/sysv/linux/seteuid.c (seteuid): Implement using
`setresuid' syscall if availble. Fixes problems with sendmail
which expects `seteuid' to not set the saved user ID.