BUG/MINOR: peers: Fix memory leak in cfg_parse_peers

BUG/MINOR: peers: Fix memory leak in cfg_parse_peers

cfg_parse_peers previously leaked the contents of the `kws` string,
as it was unconditionally filled using bind_dump_kws, but only used
(and freed) within the error case.

Move the dumping into the error case to:
1. Ensure that the registered keywords are actually printed as least once.
2. The contents of kws are not leaked.

This move allows to narrow the scope of `kws`, so this is done as well.

This bug was found using valgrind:

    ==28217== 590 bytes in 1 blocks are definitely lost in loss record 51 of 71
    ==28217==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==28217==    by 0x4AD4C7: indent_msg (standard.c:3676)
    ==28217==    by 0x47E962: cfg_parse_peers (cfgparse.c:700)
    ==28217==    by 0x480273: readcfgfile (cfgparse.c:2147)
    ==28217==    by 0x479D51: init (haproxy.c:1585)
    ==28217==    by 0x404A02: main (haproxy.c:2585)

with this super simple configuration:

    peers peers
    	bind :8081
    	server A

This bug exists since the introduction of cfg_parse_peers in commit
355b2033ec0c89660db179b23d6f77b678d8c26d (which was introduced for HAProxy
2.0, but marked as backportable). It should be backported to all branches
containing that commit.