Escape spec chars in pam password

[michael-conway]

irods log reflects unescaped special chars in pam password

Feb 14 12:19:14 pid:79334 remote addresses: 10.98.105.85, ::1 ERROR: [-]        /home/irodsbuild/irods/server/api/src/rsAuthPluginRequest.cpp:86:int rsAuthPluginR
equest(rsComm_t *, authPluginReqInp_t *, authPluginReqOut_t **) :  status [SYS_BAD_INPUT]  errno [] -- message []
        [-]     /home/irodsbuild/irods/plugins/auth/pam/libpam.cpp:392:irods::error pam_auth_agent_request(irods::plugin_context &) :  status [SYS_BAD_INPUT]  err
no [] -- message []
                [-]     /home/irodsbuild/irods/lib/core/src/irods_kvp_string_parser.cpp:136:irods::error irods::parse_escaped_kvp_string(const std::string &, kvp_
map_t &, const std::string &, const std::string &, const std::string &) :  status [SYS_BAD_INPUT]  errno [] -- message [unescaped _assoc token encountered in key 
or value]

Relating to irods/irods#3528

It appears that the escape can be done using static const std::string KVP_DEF_ESCAPE( "\" );

[michael-conway]

...replicated in test