vlogger.8: fix incorrect default -t tag documentation

[chrisdebian]

Fixes #84.

The -t tag option's man page entry claims the default tag is the LOGNAME
environment variable. That's incorrect — vlogger.c never calls getenv()
anywhere. Tracing the actual behaviour when -t isn't given:

• if /etc/vlogger is used, the tag arg passed to it defaults to an empty
  string (tag ? tag : "")
• if syslog(3) is used directly, the tag defaults to the result of
  getlogin(3) (tag ? tag : getlogin())

Neither is LOGNAME. Corrected the description to match.

(The -p pri option's documented default of user.notice is correct as-is —
facility = LOG_USER; level = LOG_NOTICE; are the literal initial values in
main() — so left that section untouched.)

Verified with groff -mandoc -Tascii -z (clean) and checked the rendered
output.

[classabbyamp]

getlogin() returns the value of $LOGNAME on musl and NULL unconditionally on glibc.

[chrisdebian]

Good catch, thanks — pushed a follow-up commit. The previous wording correctly traced the C code (tag ? tag : getlogin()) but didn't convey that the practical outcome differs by libc. Now reads:

If not given, the default is an empty string when /etc/vlogger is used. When syslog(3) is used directly, the default is the result of getlogin(3), which on glibc always returns NULL (no tag) and on musl returns the value of the LOGNAME environment variable.

Re-verified with groff -mandoc -Tascii -z (clean).