Fix handling of backlash character in LINUX_COMPILE_BY name

When using a domain login, `whoami` returns the login in
user\domain format. This leads to either warnings on unrecognised
escape sequences or escaped characters being generated for the user.
This patch ensures that any backslash is escaped to a double-backslash
to make sure the name is preserved correctly. This patch does not
enforce escaping on the KBUILD_BUILD_USER variable, as this is something
the user has control of and can escape if required.

Signed-off-by: Marcin Nowakowski <marcin.nowakowski.000@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>