fix: another attempt to avoid sigpipe on grep (#519)

Take this opportunity to factorize all the greps in /proc/cpuinfo
into a helper that avoids using a pipe to entirely avoid SIGPIPE
on a possibly gigantic /proc/cpuinfo

(cherry picked from commit 5bbffaf053)

src/vulns-helpers/check_mds.sh

@@ -138,7 +138,7 @@ check_mds_linux() {
         if is_x86_kernel; then
             pr_info_nol "* Kernel supports using MD_CLEAR mitigation: "
             kernel_md_clear_can_tell=1
-            if [ "$g_mode" = live ] && grep ^flags "$g_procfs/cpuinfo" | grep -qw md_clear; then
+            if [ "$g_mode" = live ] && cpuinfo_has_flag md_clear; then
                 kernel_md_clear="md_clear found in $g_procfs/cpuinfo"
                 pstatus green YES "$kernel_md_clear"
             fi