probe for getspnam(), forkpty(), utmpx, replace sethostname()

Android is missing all of these; we need to probe for some so we have
a config symbol to depend on.
sethostname() is easily replaced.
We got termios.h via pty.h; now it's not included in configure-step tools,
so we need termios.h to generate globals.
Fixups for the android/bionic build probes patch.

The CFG_* symbols are always defined so if() can use them as compile-time
constants, so don't if defined() them.

Doing USE_BLAH() around variable definitions opens up the same potential for
config-dependent build breaks as #ifdefs do, just make the whole command
depend on the symbol for now, factor out the utmpx infrastructure later.

The PTY probe was always failing because it used NULL without #including
the header that defines it. Substitute 0 instead.
Make md5sum and sha1sum work on big endian systems.