dnstable_dump: silence -Wsometimes-uninitialized diagnostic

this silences the following diagnostic:

src/dnstable_dump.c:126:11: warning: variable 'd_it' is used uninitialized whenever 'if' condition
      is false [-Wsometimes-uninitialized]
        else if (g_rdata)
                 ^~~~~~~
src/dnstable_dump.c:129:10: note: uninitialized use occurs here
        do_dump(d_it);
                ^~~~
src/dnstable_dump.c:126:7: note: remove the 'if' if its condition is always true
        else if (g_rdata)
             ^~~~~~~~~~~~
src/dnstable_dump.c:110:28: note: initialize the variable 'd_it' to silence this warning
        struct dnstable_iter *d_it;
                                  ^
                                   = NULL
my/zonefile: treat a second SOA as EOF

some ZFAs (e.g., .name) strangely terminate the zone file with a second
SOA, which is technically invalid per IETF and ICANN specs. add a
workaround for these broken zone files by treating a second SOA
encountered in zonefile_read() the same as EOF.

it's not necessary to actually close the FILE object in zonefile_read()
on EOF. this is performed in zonefile_destroy().
my/my_alloc: add my_free macro
my/my_alloc: add MY_ALLOC_WARN_DEPRECATED stuff wrapper functions for calloc, malloc, realloc
my/vector: free -> my_free
my/vector: mark all generated functions with the 'unused' attribute

according to the gcc documentation,

    unused

        This attribute, attached to a function, means that the function
        is meant to be possibly unused. GCC does not produce a warning
        for this function.

this makes sure the functions generated by VECTOR_GENERATE() don't
trigger -Wunused-functions diagnostics when compiled.
my/hex_decode: don't allow use-after-free in error path

in the error case, where the input passed to hex_decode() is invalid,
make sure we don't allow a stray pointer to escape via the 'raw'
parameter-return value. this prevents use-after-free() in callers.
my/m4/ld-version-script.m4: new
Merge branch 'master' of ../libmy
my_alloc updates

free -> my_free
strdup -> my_strdup
man/dnstable_query.3.txt: fix return value of dnstable_query_iter()
dnstable_convert: check return value of wdns_reverse_name()
entry: check return value of wdns_reverse_name()
query: check return value of wdns_reverse_name()
Makefile.am: drop -Wl,--as-needed from link editor flags
add symbol versioning
configure.ac: bump libwdns dependency to >= 0.6.0