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