unregister INI entries and fix invalid read on shutdown

[tony2001]

At the moment protobuf PHP extension registers an INI entry and doesn't unregister it on shtudown.
This causes invalid read if you use dl("protobuf.so") because the interned string allocated in REGISTER_INI_ENTRIES() is still present in the list of interned strings on PHP shutdown, but the extension itself is already unloaded:

==30061== Invalid read of size 4
==30061==    at 0x7420EE: zend_string_release_ex (zend_string.h:284)
==30061==    by 0x7424A5: free_ini_entry (zend_ini.c:78)
==30061==    by 0x732D05: zend_hash_destroy (zend_hash.c:1546)
==30061==    by 0x7425A1: zend_ini_dtor (zend_ini.c:113)
==30061==    by 0x742582: zend_ini_shutdown (zend_ini.c:106)
==30061==    by 0x686692: php_module_shutdown (main.c:2543)
==30061==    by 0x7F36DD: main (php_cli.c:1377)
==30061==  Address 0x81d63f4 is 4 bytes inside a block of size 72 free'd
==30061==    at 0x4C2F50B: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==30061==    by 0x7585D9: free (zend_signal.c:109)
==30061==    by 0x757B0F: _str_dtor (zend_string.c:60)
==30061==    by 0x732CC7: zend_hash_destroy (zend_hash.c:1541)
==30061==    by 0x75834E: zend_interned_strings_deactivate (zend_string.c:307)
==30061==    by 0x68575A: php_request_shutdown (main.c:1991)
==30061==    by 0x7F2F26: do_cli (php_cli.c:1129)
==30061==    by 0x7F367B: main (php_cli.c:1362)
==30061==  Block was alloc'd at
==30061==    at 0x4C2E2DF: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==30061==    by 0x758507: malloc (zend_signal.c:91)
==30061==    by 0x6E481A: __zend_malloc (zend_alloc.c:2976)
==30061==    by 0x75745F: zend_string_alloc (zend_string.h:133)
==30061==    by 0x7574FC: zend_string_init (zend_string.h:155)
==30061==    by 0x7582E7: zend_string_init_interned_request (zend_string.c:293)
==30061==    by 0x742846: zend_register_ini_entries (zend_ini.c:230)
==30061==    by 0xA58B9FE: zm_startup_protobuf (protobuf.c:290)
==30061==    by 0x7241DA: zend_startup_module_ex (zend_API.c:1868)
==30061==    by 0x5CE3C1: php_load_extension (dl.c:232)
==30061==    by 0x5CE47C: php_dl (dl.c:253)
==30061==    by 0x5CDD83: zif_dl (dl.c:64)

Unregistering the INI entries properly frees this interned string in the correct way and fixes this issue.