The routines necessary for implementing Wagner's API for dropping
privileges are:

sysconf function: available in os module
_SC_NGROUPS_MAX constant: unsure
abort function: available in os module
getresuid function: needs implementing[1]
getresgid function: needs implementing[1]

[1] Not POSIX but available on Linux, HP/UX, FreeBSD, OpenBSD,
DragonFlyBSD.  Not available on Solaris, AIX, NetBSD, OSX, cygwin.

Since Wagner points out that the non-standard function calls actually
have clearer/better semantics, perhaps the way to do all this is in its
own module, instead of trying to shove it into an existing module..