Mirror the r26.1.0 Emergency Bug Fix (changes 33608, 33609) into
main: 5 scripts and 1 systemd unit hardcoded the planned 2026.2 OS
package directory name (/opt/perforce/p4-sdp) instead of the
current 2026.1 name (/opt/perforce/helix-sdp) --
sdp_upgrade.sh, opt_perforce_sdp_backup.sh (+ its systemd unit),
install_sdp.sh, mkdirs.sh, and configure_sample_depot_for_sdp.sh.
install_sdp.sh is the actual customer-facing installer, so every
fresh package-structure install of 2026.1 was creating the wrong
top-level directory. Also fixed install_sdp.sh's BackupScript
variable (hardcoded the same wrong path instead of deriving from
SDPPackageBase, feeding into SELinux fcontext/restorecon calls),
mkdirs.sh's shipped -man/-h usage text, and several cosmetic
source comments.