This way the man pages are installed only when the corresponding binary is
installed. The conditions in man pages and man/rules/meson.build are adjusted to
match the conditions for units in units/meson.build.
The binaries are built and installed if HAVE_TPM2 is set, and ignore ENABLE_BOOTLOADER,
so do the same for the manpages.
For the sd-pcrlock case this also installs the manpage aliases for the units, which
are not installed with -Dbootloader=disabled, but there's no way to conditionalize
the aliases, so on balance it's better to have too much documentation rather than
too little.
Fixes https://github.com/systemd/systemd/issues/30588
