When a SOCKS5 client sends a RESOLVE_PTR request, it must include
either an IPv4 or IPv6 address. In the past this was required to be a
binary address (address types 1 or 4), but since the refactoring of
SOCKS5 support in Tor 0.3.5.1-alpha, strings (address type 3) are also
allowed if they represent an IPv4 or IPv6 literal.
However, when a binary IPv6 address is provided,
parse_socks5_client_request converts it into a string enclosed in
brackets. This doesn't match what string_is_valid_ipv6_address
expects, so this would fail with the error "socks5 received
RESOLVE_PTR command with hostname type. Rejecting."
By replacing string_is_valid_ipv4_address/string_is_valid_ipv6_address
with tor_addr_parse, we accept strings both with and without brackets.
This fixes the handling of binary addresses, and also improves
symmetry with CONNECT and RESOLVE requests.
Fixes bug 32315.
