Fix a memory leak in py-param.c

Fix a memory leak in py-param.c

Mark Wielaard pointed out this memory leak to me:

    ==17633== 775 bytes in 1 blocks are definitely lost in loss record 13,346 of 13,967
    ==17633==    at 0x4C2DB6B: malloc (vg_replace_malloc.c:299)
    ==17633==    by 0x6652B7: xmalloc (common-utils.c:45)
    ==17633==    by 0xC4C889: xstrdup (xstrdup.c:34)
    ==17633==    by 0x5A71FD: unicode_to_encoded_string(_object*, char const*) (py-utils.c:81)
    ==17633==    by 0x5A73EB: python_string_to_host_string(_object*) (py-utils.c:158)
    ==17633==    by 0x59CC6C: get_doc_string(_object*, _object*) (py-param.c:334)
    ==17633==    by 0x59D2AA: parmpy_init(_object*, _object*, _object*) (py-param.c:728)

The bug here is that parmpy_init is written as though
add_setshow_generic takes ownership of its doc-string arguments.
However, it does not.  This patch fixes the bug in a straightforward
way and also applies some missing constification to make the problem
more apparent.

Tested on x86-64 Fedora 26.

gdb/ChangeLog
2018-06-20  Tom Tromey  <tom@tromey.com>

	* python/py-param.c (add_setshow_generic): Make parameters const.
	(parmpy_init): Update.