log: add Lmsgprefix flag

The Lmsgprefix flag moves the logger's prefix from the
beginning of the line to after the log header. For example,
a logger with the prefix "LOG " and LstdFlags would output:

    LOG 2009/11/10 23:00:00 entry text

Adding the Lmsgprefix flag would output:

    2009/11/10 23:00:00 LOG entry text

Fixes #32062

Change-Id: I9f7c9739abeb53c424112aaeed33444eeefdfbbc
Reviewed-on: https://go-review.googlesource.com/c/go/+/186182


Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>