libkmod: honor install commands on modules with softdeps

[prabhakarpujeri]

Fixes #446.

Since 89e9248 ("modprobe: obey blacklist and softdep pre"), __kmod_module_fill_softdep() marks every module that has any softdep entry with ignorecmd, and kmod_module_probe_insert_module() then silently skips the module's configured install command and falls back to insmod.

Consequence: the documented block formula

blacklist ksmbd
install ksmbd /bin/false

has no effect for a module that ships a softdep (Gentoo confirmed blacklist+install is silently ignored) – a dry run inserts the real module file, and as the reporter notes, "even something as simple as a softdep kills any install command".

Changes

• libkmod: only set ignorecmd when no install command is configured (kmod_module_get_install_commands(mod) == NULL). An explicit install directive is user configuration and must win over softdep metadata; modules without one keep the existing behavior, and --ignore-install is unaffected.
• testsuite: new fixture test-modprobe/install-cmd-softdep – module with a pre softdep plus install mod-foo /bin/true. The dry run must print insmod …/mod-foo-a.ko followed by install /bin/true; without the fix it prints insmod …/mod-foo.ko instead. Verified red on master, green with this change; full testsuite passes.

Reproduction (on master)

# fake rootfs: mod-foo.ko + mod-foo-a.ko,
# modules.softdep: softdep mod-foo pre: mod-foo-a
# /run modprobe.d: blacklist mod-foo / install mod-foo /bin/false
$ modprobe -n -v mod-foo
insmod <rootfs>/lib/modules/$(uname -r)/kernel/mod-foo-a.ko
insmod <rootfs>/lib/modules/$(uname -r)/kernel/mod-foo.ko   ← installs despite /bin/false

after this change, the second line is install /bin/false.