core: change mount options settings so that last defined wins (#39449)

diff --cc man/systemd.exec.xml
Simple merge

diff --cc src/core/dbus-execute.c
Simple merge

diff --cc src/core/execute-serialize.c
Simple merge

diff --cc src/core/execute.c
Simple merge

diff --cc src/core/execute.h
Simple merge

diff --cc src/core/load-fragment.c
Simple merge

diff --cc src/core/namespace.c
Simple merge

diff --cc src/core/namespace.h
Simple merge

diff --cc src/core/varlink-execute.c
Simple merge