# cd /usr/src/9

# mergemaster -p
*** Creating the temporary root environment in /var/tmp/temproot
 *** /var/tmp/temproot ready for use
 *** Creating and populating directory structure in /var/tmp/temproot

cp: /usr/src/etc/master.passwd: No such file or directory

  *** FATAL ERROR: Cannot copy files to the temproot environment

WORKAROUND FOR THIS:
#cd /usr/src
#ln -s /usr/src/9/etc /usr/src/etc

After that I have next FATAL:
# mergemaster
*** Creating the temporary root environment in /var/tmp/temproot
 *** /var/tmp/temproot ready for use
 *** Creating and populating directory structure in /var/tmp/temproot

make: don't know how to make distrib-dirs. Stop

  *** FATAL ERROR: Cannot 'cd' to /usr/src and install files to
      the temproot environment

WORKAROUND FOR THIS:
#cd /usr
#mv /usr/src/ /usr/srca
#ln -s /usr/srca/9 /usr/src
#ln -s /usr/srca/9 usr/src/9


Please FIX that and allow to build/install world while sources are not in /usr/src

EXPECTED problem is here:
/usr/src/usr.sbin/mergemaster/mergemaster.sh:638
# Assign the source directory
#
SOURCEDIR=${SOURCEDIR:-/usr/src} <<<<<<<<<<<<<<<<<<<<<
if [ ! -f ${SOURCEDIR}/Makefile.inc1 -a \
   -f ${SOURCEDIR}/../Makefile.inc1 ]; then
  echo " *** The source directory you specified (${SOURCEDIR})"

However doing:
-SOURCEDIR=${SOURCEDIR:-/usr/src}
+SOURCEDIR=${SOURCEDIR:-/usr/src/9}
while I were in /usr/src/9 do not fix the problem.
So I did symlinks.