ping: install a pong handler

Respond to server pings:

https://xmpp.org/extensions/xep-0199.html#s2c

```
xmpp DEBUG RECV: <iq id="rr-1694698253331-17375542133555098656-wf3J7kbG6j8Ft5Vx0cMY99txsW8=-55238004" type="get" to="[email protected]/110052433108464573191602" from="example.com"><ping xmlns="urn:xmpp:ping"/></iq>
conn DEBUG SENT: <iq id="rr-1694698253331-17375542133555098656-wf3J7kbG6j8Ft5Vx0cMY99txsW8=-55238004" to="example.com" type="result" from="[email protected]/110052433108464573191602"/>
```

Thanks @jessiehowell !

Fixes #9

Replace ctype usage

The ctype functions are error prone. The argument is an int which must
be in the range of an unsigned char and is interpreted based on the
locale:

    These functions check whether c, which must have the value of
    an unsigned char or EOF, falls into a certain character class
    according to the  specified  locale. The  functions without the
    "_l" suffix perform the check based on the current locale.

capsicum: disable fs writes

Use RLIMIT_FSIZE to disable writes to the filesystem when stdout is not
a regular file.

0.14.5: fix usage

0.14.3: seccomp: add syscalls

Add syscalls for Ubuntu 22.04.

0.14.2

Tag release for #6

Thanks @anjandev!