mtbl_merge: fix tiny memory leak in init_dso()

"func_name" is a heap allocated object local to the init_dso() function,
so it needs to be freed before the function returns.
sorter: _mtbl_sorter_write_chunk(): free allocated chunk object on error return

the variable 'c' is allocated in the _mtbl_sorter_write_chunk() function
and leaks on an error return. detected by:

mtbl/sorter.c:199:6: warning: Potential leak of memory pointed to by 'c'
                                        mtbl_writer_destroy(&w);
                                        ^~~~~~~~~~~~~~~~~~~
sorter: mtbl_sorter_iter(): free allocated iter object on error return

the variable 'it' is allocated in the mtbl_sorter_iter() function and
leaks on an error return. detected by:

mtbl/sorter.c:320:12: warning: Potential leak of memory pointed to by 'it'
                        return (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
my/my_fileset: my_fileset_reload(): don't leak on error return
reader: mtbl_reader_init_fd(): don't leak 'fd' on error return
test-vector: test6(): don't leak 'v'
fileset: fs_reinit_merger(): silence coverity
Merge branch 'master' of ../libmy
Makefile.am: drop -Wl,--as-needed from link editor flags
bump version to 0.6.0, add symbol versioning