Fix socks4 and socks4a support under glibc's source fortification

Fix socks4 and socks4a support under glibc's source fortification

With glib'c source fortification, gcc offers the compilation warning

    gateway.c: In function 'socks4_connect':
    gateway.c:840:4: warning: 'strlcpy' writing 4988 bytes into a region of size 1 overflows the destination
      840 |    strlcpy(&(c->userid), socks_userid, sizeof(buf) - sizeof(struct socks_op));
	  |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    gateway.c:112:9: note: destination object 'userid' of size 1
      112 |    char userid;               /* first byte of userid */
	  |         ^~~~~~

resulting in a runtime abort() when using a socks4 or socks4a upstream proxy:

    $ privoxy --no-daemon <(echo 'forward-socks4 / 127.0.0.1:9050 .')
    7fef77483740 Info: Privoxy version 3.0.34
    7fef77483740 Info: Program name: privoxy
    *** buffer overflow detected ***: terminated
    rc: line 3: 321835 Aborted    (core dumped) privoxy --no-daemon <(echo 'forward-socks4 / 127.0.0.1:9050 .')

Despite the warning, the strlcpy() call in question is fine: gcc
misidentifies the size of the destination buffer, estimating to hold
only a single char while in fact the buffer stretches beyond the end of
the struct socks_op.

This commit fixes this issue in a way which is in line with the second
strlcpy() call in the socks4_connect(). Alternatively, we could also
remove the padding member and promote userid to a trailing flexible array
member. However, this would necessitate further adjustments because that
way the size of struct socks_op would change.

The issue was originally reported in the NixOS issue tracker at

    https://github.com/NixOS/nixpkgs/issues/265654

prompted by an upgrade of glibc from 2.37-39 to 2.38-0, and the fix is
joint work with @esclear and @richi235.