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

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.