Setting the effective user (group) ID to the saved user (group) ID is possible since Linux 1.1.37 (1.1.38). On an arbitrary system one should check _POSIX_SAVED_IDS.
Under libc4, libc5 and glibc2.0 seteuid(euid) is equivalent to setreuid(-1," euid") and hence may change the saved user ID. Under glibc2.1 it is equivalent to setresuid(-1," euid",-1) and hence does not change the saved user ID. Similar remarks hold for setegid.
geteuid(2), setuid(2), setreuid(2), setresuid(2)