exceptions: hack to store errno in string object.

Basic idea: when we throw an exception that pertains to a
system error which has an errno code, we can stick the errno
into the memory area of the character string, into the wchar_t
that immediately follows the null terminator. We can do this
because strings track their actual allocation size.

A pair of setter/getter functions to set and retrieve this
value are provided, and all functions in the code which can
set such a code are updated to do so, simply by calling the
newly added uw_ethrowf that drop-in replaces for uw_throwf.

* lib.[ch] (string_set_code, string_get_code): New functions.

* unwind.[ch] (uw_ethrowf): New function.

* eval.c (eval_init): Register string-set-code and
string-get-code intrinsics.

* ftw.c (ftw_wrap): Switch to uw_ethrowf.

* parser.c (open_txr_file): Likewise.

* socket.c (dgram_overflow): Store the ENOBUFS error in errno,
and use uw_ethrowf instead uw_throwf.
(dgram_get_byte_callback, dgram_flush, sock_bind, to_connect,
open_sockfd, sock_connect, sock_listen, sock_accept,
sock_shutdown, sock_timeout, socketpair_wrap): Switch to
uw_ethrowf.

* stream.c (dev_null_get_fd, stdio_maybe_read_error,
stdio_maybe_error, stdio_close, pipe_close, open_directory,
open_file, open_fileno, open_tail, fds_subst,
open_subprocess, open_command, remove_path, rename_path,
tmpfile_wrap, mkdtemp_wrap, mkstemp_wrap): Switch to uw_ethrowf.

* sysif.c (mkdir_wrap, ensure_dir, chdir_wrap, getcwd_wrap,
rmdir_wrap, mknod_wrap, mkfifo_wrap, chmod_wrap, do_chown,
symlink_wrap, link_wrap, readlink_wrap, close_wrap, val
exec_wrap, stat_impl, do_utimes, pipe_wrap, poll_wrap,
getgroups_wrap, setuid_wrap, seteuid_wrap, setgid_wrap,
setegid_wrap, setgroups_wrap, getresuid_wrap, setresuid_wrap,
setresgid_wrap, crypt_wrap, uname_wrap, opendir_wrap,
getrlimit_wrap, setrlimit_wrap): Likewise.

* termios.c (tcgetattr_wrap, tcsetattr_wrap, tcsendbreak_wrap,
tcdrain_wrap, tcflush_wrap, tcflow_wrap): Likewise.

* tests/018/errno.tl: New file.

* txr.1: Documented.

* stdlib/doc-syms.tl: Updated.

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