kargs: Fix crash on quoted values in /proc/cmdline

kargs: Fix crash on quoted values in /proc/cmdline

Bootloaders like GRUB may reformat quoted kernel arguments, producing
entries like "testparam=value with spaces" in /proc/cmdline. The
proc-cmdline parser was still using a naive space split (pre-dating the
quote-aware splitter added in abc7d5b9), so these tokens got broken
apart and hit a g_assert_false() abort downstream.

Use the quote-aware splitter for /proc/cmdline too, and downgrade the
unterminated-quote assert to g_debug() since we can't control what
bootloaders emit.

Reported by Arch Linux maintainers building ostree 2026.1.

Closes: https://github.com/ostreedev/ostree/issues/3582
Assisted-by: OpenCode (Claude Opus 4.6)
Signed-off-by: Joseph Marrero Corchado <jmarrero@redhat.com>