openrc-init: fix buffer overflow in init.ctl

How to reproduce 1-byte overflow:

```
$ FEATURES=-test CFLAGS="-fsanitize=address -O0 -ggdb3" emerge -1 openrc

=================================================================
==1==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7fff0efd8710
    at pc 0x000000402076 bp 0x7fff0efd7d50 sp 0x7fff0efd7d40
WRITE of size 1 at 0x7fff0efd8710 thread T0
    #0 0x402075  (/sbin/openrc-init+0x402075)
    #1 0x3cf6e2070f in __libc_start_main (/lib64/libc.so.6+0x3cf6e2070f)
    #2 0x4013b8  (/sbin/openrc-init+0x4013b8)

Address 0x7fff0efd8710 is located in stack of thread T0 at offset 2432 in frame
    #0 0x401cfb  (/sbin/openrc-init+0x401cfb)

  This frame has 3 object(s):
    [32, 160) 'signals'
    [192, 344) 'sa'
    [384, 2432) 'buf' <== Memory access at offset 2432 overflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow ??:0 ??
```

The problem here is in the code handling reads from 'init.ctl':

```
int main(int argc, char **argv) {
...
    char buf[2048];
    for (;;) {
        /* This will block until a command is sent down the pipe... */
        fifo = fopen(RC_INIT_FIFO, "r");
        count = fread(buf, 1, 2048, fifo);
        buf[count] = 0;
        ...
    }
```

`buf[count] = 0;` writes outside the buffer when `fread()` returns non-truncated read.

This fixes #138.

src/rc/openrc-init.c

@@ -195,7 +195,7 @@ int main(int argc, char **argv)
 				perror("fopen");
 			continue;
 		}
-		count = fread(buf, 1, 2048, fifo);
+		count = fread(buf, 1, sizeof(buf) - 1, fifo);
 		buf[count] = 0;
 		fclose(fifo);
 		printf("PID1: Received \"%s\" from FIFO...\n", buf);