Fix Tcl_TraceVar() / tcl_eggstr() modifying original tcl variable

[michaelortmann]

Found by: DasBrain
Patch by: michaelortmann
Fixes: #1544

One-line summary:
Fix Tcl_TraceVar() / tcl_eggstr()

Additional description (if needed):
This bug affects eggdrop since the dawn of time.

Eggdrop uses Tcl_TraceVar() when a tcl variable is set. In callback function tcl_eggstr() it truncates strings to their maximum length. The Bug is, that it modifies the original string here:

eggdrop/src/tcl.c

Line 287 in 0f5599e

s[abs(st->max)] = 0;

This PR fixes this, by modifying the destination string, when the original is copied to the destination here:

eggdrop/src/tcl.c

Line 299 in 0f5599e

strcpy(st->str, s);

via replacing that strcpy() with a strlcpy().

There are a few special variables / handlers here:

eggdrop/src/tcl.c

Lines 288 to 298 in 0f5599e

	if (st->str == botnetnick)
	botnet_change(s);
	else if (st->str == logfile_suffix)
	logsuffix_change(s);
	else if (st->str == firewall) {
	splitc(firewall, s, ':');
	if (!firewall[0])
	strcpy(firewall, s);
	else
	firewallport = atoi(s);
	} else

Test cases demonstrating functionality (if applicable):
eggdrop.conf:

set dasbrain's-awesome-configuration-template-name tcl9eggdrop
putlog "DBG: dasbrain's-awesome-configuration-template-name: ${dasbrain's-awesome-configuration-template-name}"
set username ${dasbrain's-awesome-configuration-template-name}
putlog "DBG: Username: $username"
putlog "DBG: dasbrain's-awesome-configuration-template-name: ${dasbrain's-awesome-configuration-template-name}"

Before:

DBG: dasbrain's-awesome-configuration-template-name: tcl9eggdrop
WARNING: Value for username truncated to 10 chars
DBG: Username: tcl9eggdro
DBG: dasbrain's-awesome-configuration-template-name: tcl9eggdro

After:

DBG: dasbrain's-awesome-configuration-template-name: tcl9eggdrop
WARNING: Value for username truncated to 10 chars
DBG: Username: tcl9eggdro
DBG: dasbrain's-awesome-configuration-template-name: tcl9eggdrop