free: reusing 'shared' for Shmem

Previously the shared memory column was always zero
for 2.6 series kernels (and later) due to the fact,
that the value was taken from the MemShared entry
that disappeared with 2.6 series kernels.
Later a new Shmem entry appeared in the /proc/meminfo
file and the 'shared' column now displays either
the MemShared or the Shmem value (depending on their
presence - the presence is mutually exclusive).
If none of the two entries is exported by the kernel,
then the column is zero.

proc/sysinfo.c

@@ -629,13 +629,14 @@ void meminfo(void){
   {"LowTotal",     &kb_low_total},
   {"Mapped",       &kb_mapped},       // kB version of vmstat nr_mapped
   {"MemFree",      &kb_main_free},    // important
-  {"MemShared",    &kb_main_shared},  // important, but now gone!
+  {"MemShared",    &kb_main_shared},  // obsolete since kernel 2.6! (sharing the variable with Shmem replacement)
   {"MemTotal",     &kb_main_total},   // important
   {"NFS_Unstable", &kb_nfs_unstable},
   {"PageTables",   &kb_pagetables},   // kB version of vmstat nr_page_table_pages
   {"ReverseMaps",  &nr_reversemaps},  // same as vmstat nr_page_table_pages
   {"SReclaimable", &kb_swap_reclaimable}, // "swap reclaimable" (dentry and inode structures)
   {"SUnreclaim",   &kb_swap_unreclaimable},
+  {"Shmem",        &kb_main_shared},  // kernel 2.6 and later (sharing the output variable with obsolete MemShared)
   {"Slab",         &kb_slab},         // kB version of vmstat nr_slab
   {"SwapCached",   &kb_swap_cached},
   {"SwapFree",     &kb_swap_free},    // important