C99: Mark POSIX source files

Specifly POSIX standard for every source
file that needs it.

Hide term_t structure inside src/io/term.c
not to expose POSIX (struct timespec) to client.

Remove unnecessary #include "io/term.h"
Merge branch 'strdup' into c99
cfg_parser.y does not use fileno(2)
Get rid of POSIX fileno(2) in flex output

Flex generated code checks if the input
file is a terminal using isatty(2) and fileno(2).

We don't need that as we always pass either string
buffer or a disk file.