libc: Add gets_s(3) from ISO/IEC TR 24731-1.

For more information see the Technical Report (drafts of it are on
the web) or e.g. http://en.cppreference.com/w/c/io/gets.

This also adds set_constrain

libc: Add gets_s(3) from ISO/IEC TR 24731-1.

For more information see the Technical Report (drafts of it are on
the web) or e.g. http://en.cppreference.com/w/c/io/gets.

This also adds set_constraint_handler_s(), abort_handler_s(),
ignore_handler_s(), the RSIZE_MAX define, and the types rsize_t,
errno_t and constraint_handler_t.

Visibility is restricted to our default environment by default, but
can be enabled by defining __STDC_WANT_LIB_EXT1__ to 1, per the TR.

While here, put gets()'s documentation into a manual page of its own
with a note that it is depracated. We'll likely remove it from libc
too at a later point in time.

Requested-by: zrj
Taken-from: FreeBSD (with some adjustments)

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