Fix parsing bug in linux get_total_system_memory().

Use find_str_at_start_of_line(), not strstr() here: we don't want
to match "MemTotal: " if it appears in the middle of a line.

Fixes #40315; bugfix on 0.2.5.4-alpha.

changes/bug40315

+  o Minor bugfixes (Linux, relay):
+    - Fix a bug in determining total available system memory that would have
+      been triggered if the format of /proc/meminfo had ever changed
+      to include "MemTotal:" in the middle of a line. Fixes bug 40315;
+      bugfix on 0.2.5.4-alpha.

src/lib/meminfo/.may_include

@@ -5,4 +5,5 @@ lib/fs/*.h
 lib/log/*.h
 lib/malloc/*.h
 lib/meminfo/*.h
+lib/string/*.h
 lib/testsupport/*.h

src/lib/meminfo/meminfo.c

@@ -17,6 +17,7 @@
 #include "lib/fs/files.h"
 #include "lib/log/log.h"
 #include "lib/malloc/malloc.h"
+#include "lib/string/util_string.h"
 
 #ifdef HAVE_FCNTL_H
 #include <fcntl.h>
@@ -65,7 +66,7 @@ get_total_system_memory_impl(void)
   s = read_file_to_str_until_eof(fd, 65536, &file_size);
   if (!s)
     goto err;
-  cp = strstr(s, "MemTotal:");
+  cp = find_str_at_start_of_line(s, "MemTotal:");
   if (!cp)
     goto err;
   /* Use the system sscanf so that space will match a wider number of space */