simplify ctermid

opening /dev/tty then using ttyname_r on it does not produce a
canonical terminal name; it simply yields "/dev/tty".

it would be possible to make ctermid determine the actual controlling
terminal device via field 7 of /proc/self/stat, but doing so would
introduce a buffer overflow into applications built with L_ctermid==9,
which glibc defines, adversely affecting the quality of ABI compat.

include cleanups: remove unused headers and add feature test macros

fix bogus lazy allocation in ctermid and missing malloc failure check

also clean up, optimize, and simplify the code, removing branches by
simply pre-setting the result string to an empty string, which will be
preserved if other operations fail.

fix fd leak on races and cancellation in ctermid

initial check-in, version 0.5.0