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.c

@@ -139,3 +139,20 @@ pscanf(const char *path, const char *fmt, ...)
 
 	return (n == EOF) ? -1 : n;
 }
+
+int
+lscanf(FILE *fp, const char *key, const char *fmt, void *res)
+{
+		int n;
+		char line[256];
+
+		n = -1;
+		while (fgets(line, sizeof(line), fp))
+			if (strncmp(line, key, strlen(key)) == 0) {
+				n = sscanf(line + strlen(key), fmt, res);
+				break;
+			}
+
+		rewind(fp);
+		return (n == 1) ? 1 : -1;
+}