libc: Some enhancements to syslog(3)

- Defined MAXLINE constant (8192 octets by default instead 2048) for
  centralized limit setting up. It sets maximum number of characters of
  the syslog message. RFC5424 doesn't limit maximum size of the message.
  Named after MAXLINE in syslogd(8).
- Fixed size of fmt_cpy buffer up to MAXLINE for rendering formatted
  (%m) messages.
- Introduced autoexpansion of sending socket buffer up to MAXLINE.

MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D27205

lib/libc/gen/syslog.c

@@ -57,6 +57,9 @@ __FBSDID("$FreeBSD$");
 
 #include "libc_private.h"
 
+/* Maximum number of characters of syslog message */
+#define	MAXLINE		8192
+
 static int	LogFile = -1;		/* fd for log */
 static int	status;			/* connection status */
 static int	opened;			/* have done openlog() */
@@ -141,7 +144,7 @@ vsyslog1(int pri, const char *fmt, va_list ap)
 	char ch, *p;
 	long tz_offset;
 	int cnt, fd, saved_errno;
-	char hostname[MAXHOSTNAMELEN], *stdp, tbuf[2048], fmt_cpy[1024],
+	char hostname[MAXHOSTNAMELEN], *stdp, tbuf[MAXLINE], fmt_cpy[MAXLINE],
 	    errstr[64], tz_sign;
 	FILE *fp, *fmt_fp;
 	struct bufcookie tbuf_cookie;
@@ -396,9 +399,19 @@ connectlog(void)
 	struct sockaddr_un SyslogAddr;	/* AF_UNIX address of local logger */
 
 	if (LogFile == -1) {
+		socklen_t len;
+
 		if ((LogFile = _socket(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC,
 		    0)) == -1)
 			return;
+		if (_getsockopt(LogFile, SOL_SOCKET, SO_SNDBUF, &len,
+		    &(socklen_t){sizeof(len)}) == 0) {
+			if (len < MAXLINE) {
+				len = MAXLINE;
+				(void)_setsockopt(LogFile, SOL_SOCKET, SO_SNDBUF,
+				    &len, sizeof(len));
+			}
+		}
 	}
 	if (LogFile != -1 && status == NOCONN) {
 		SyslogAddr.sun_len = sizeof(SyslogAddr);