clang generates internal calls for some _chk symbol, so add internal
aliases for them, and stub some with rtld-stubbed-symbols to avoid
ld.so linker issues.

Reviewed-by: Sam James <sam@gentoo.org>

clang issues:

syslog.c:193:9: error: adding 'int' to a string does not append to the string [-Werror,-Wstring-plus-int]
  193 |                       SYSLOG_HEADER (pri, timestamp, &msgoff, pid));
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
syslog.c:180:7: note: expanded from macro 'SYSLOG_HEADER'
  180 |   "[" + (pid == 0), pid, "]" + (pid == 0)

Use array indexes instead of string addition (it is simpler than
add a supress warning).

__vsyslog_internal calculated a buffer size by adding two integers, but
did not first check if the addition would overflow.  This commit fixes
that.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>

__vsyslog_internal used the return value of snprintf/vsnprintf to
calculate buffer sizes for memory allocation.  If these functions (for
any reason) failed and returned -1, the resulting buffer would be too
small to hold output.  This commit fixes that.

All snprintf/vsnprintf calls are checked for negative return values and
the function silently returns upon encountering them.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

__vsyslog_internal did not handle a case where printing a SYSLOG_HEADER
containing a long program name failed to update the required buffer
size, leading to the allocation and overflow of a too-small buffer on
the heap.  This commit fixes that.  It also adds a new regression test
that uses glibc.malloc.check.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>

This allows to include bits/syslog-decl.h in include/sys/syslog.h and
therefore be able to create the libc_hidden_builtin_proto (__syslog_chk)
prototype.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

The rfc3164 clear states that a single space character must follow
the timestamp field.

Checked on x86_64-linux-gnu.

The a583b6add407c17cd change did not handle large messages that
would require a heap allocation correctly, where the message itself
is not take in consideration.

This patch fixes it and extend the tst-syslog to check for large
messages as well.

Checked on x86_64-linux-gnu.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

It also handles the highly unlikely case where localtime might return
NULL, in this case only the PRI is set to hopefully instruct the relay
to get eh TIMESTAMP (as defined by the RFC).

Checked on x86_64-linux-gnu and i686-linux-gnu.

A fixed-sized buffer is used instead of memstream for messages up to
1024 bytes to avoid the potential BUFSIZ (8K) malloc and free for
each syslog call.

Also, since the buffer size is know, memstream is replaced with a
malloced buffer for larger messages.

Checked on x86_64-linux-gnu.