SDP-1379: implement the mount-point-naming part (mkdirs.sh/mkdirs.cfg
only -- install_sdp.sh generates its own config and doesn't use these).
- mkdirs.cfg: update real (non-test) default mount points to the new
SDP-1379 convention: DD/CD /p4depots -> /mnt/p4depots, LG /p4logs ->
/mnt/p4logs, DB1/DB2 /p4db -> /mnt/p4db.
- mkdirs.sh functional fix: '-test' mode's SDPMountPointBase was still
hardcoded to /tmp/hxmounts; changed to /tmp/p4mounts, matching what
Server/test/test_SDP.py's mkdirsTest already checks for.
- mkdirs.sh structural fix: -test/-R mode's path substitution only
worked by accident when DD/CD/LG/DB1/DB2 had exactly one path
component. Fixed to take just the basename of each real mount point
when staging, robust regardless of path depth.
- mkdirs.sh comments/doc text updated to match the /mnt/p4* convention.