more concise memory calculation on Linux

more flexible parsing for /proc/meminfo to take shared and reclaimable
memory into account. this matches the output with free(1).

additionally this could fix some corner cases, as the order of fields in
/proc/meminfo is not strictly defined.

slstatus:
percent 81% free 2.5 Gi total 23.4 Gi used 19.0 Gi

free(1):
               total        used        free      shared  buff/cache   available
Mem:            23Gi        19Gi       2.5Gi       1.3Gi       3.2Gi       3.6Gi

util.h

@@ -1,5 +1,6 @@
 /* See LICENSE file for copyright and license details. */
 #include <stdint.h>
+#include <stdio.h>
 
 extern char buf[1024];
 
@@ -14,3 +15,4 @@ int esnprintf(char *str, size_t size, const char *fmt, ...);
 const char *bprintf(const char *fmt, ...);
 const char *fmt_human(uintmax_t num, int base);
 int pscanf(const char *path, const char *fmt, ...);
+int lscanf(FILE *fp, const char *key, const char *fmt, void *res);