Don't assume `tcdrain`/`ctermid` exists everywhere

[hvr]

Turns out that some platforms (e.g. versions prior to API level 21 of the Android API, as reported via #54, as can be seen from https://github.com/android/platform_bionic/blob/master/libc/include/termios.h + https://github.com/android/platform_bionic/blob/master/libc/include/android/legacy_termios_inlines.h) are lacking at least the POSIX.1-2001 termios(3) functions

• tcdrain(3) and
• ctermid(3)

However, unix currently assumes those are available unconditionally