Fix information leak through TCL variables.

[michaelortmann]

Found by: maimizuno
Patch by: michaelortmann
Fixes: #137 and #414 as part of #51

One-line summary:
Fix information leak through TCL variables.

Additional description (if needed):
Before check_tcl_bind() Tcl_SetVar() is called. This Fix adds Tcl_UnsetVar() at the end of check_tcl_bind()

Please don't forget to update (not close) #51 after merge.

Test cases demonstrating functionality (if applicable):

1. scripts/643.tcl

proc 643 args {
  foreach a [info vars ::_*] { if ![string eq "" [set $a]] { putlog "DEBUG $a:[set $a]" } }
  return 0
}

2. eggdrop.conf

source scripts/643.tcl

3. $ ./eggdrop -nt

4. send private message to eggdrop

/msg <botnick> testmsg

5. execute at partyline:

.tcl 643

before:

[01:00:35] [testuser!~michael@localhost] testmsg 
.tcl 643
[01:00:41] DEBUG ::_lu1:assoc
[01:00:41] DEBUG ::_event1:init-server
[01:00:41] DEBUG ::_out1:server
[01:00:41] DEBUG ::_out2:WHOIS linux
[01:00:41] DEBUG ::_out3:sent
[01:00:41] DEBUG ::_log1:o
[01:00:41] DEBUG ::_log2:*
[01:00:41] DEBUG ::_log3:DEBUG ::_log2:*
[01:00:41] DEBUG ::_filt1:1
[01:00:41] DEBUG ::_filt2:.tcl 643
[01:00:41] DEBUG ::_msg1:testuser
[01:00:41] DEBUG ::_msg2:~michael@localhost
[01:00:41] DEBUG ::_msg3:michael
[01:00:41] DEBUG ::_msgm1:testuser
[01:00:41] DEBUG ::_msgm2:~michael@localhost
[01:00:41] DEBUG ::_msgm3:michael
[01:00:41] DEBUG ::_msgm4:testmsg
[01:00:41] DEBUG ::_raw1:zen.localdomain
[01:00:41] DEBUG ::_raw2:318
[01:00:41] DEBUG ::_raw3:linux linux :End of WHOIS list.
[01:00:41] DEBUG ::_dcc1:-HQ
[01:00:41] DEBUG ::_dcc2:1
[01:00:41] DEBUG ::_dcc3:643
[01:00:41] DEBUG ::_chonof1:-HQ
[01:00:41] DEBUG ::_chonof2:1
[01:00:41] DEBUG ::_chjn1:linux
[01:00:41] DEBUG ::_chjn2:-HQ
[01:00:41] DEBUG ::_chjn3:0
[01:00:41] DEBUG ::_chjn4:*
[01:00:41] DEBUG ::_chjn5:1
[01:00:41] DEBUG ::_chjn6:llama@console
Tcl: 0

after:

[01:02:14] [testuser!~michael@localhost] testmsg 
.tcl 643
[01:02:17] DEBUG ::_dcc1:-HQ
[01:02:17] DEBUG ::_dcc2:1
[01:02:17] DEBUG ::_dcc3:643
Tcl: 0

[Cizzle]

LGTM

[vanosg]

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