This function returns the level of the specified skill that the invoking
character has. If they don't have the skill, 0 will be returned. The full list
of character skills is available in 'db/skill_db.txt'.

There are two main uses for this function, it can check whether the character
has a skill or not, and it can tell you if the level is high enough.

Example 1:

if (getskilllv(152)) goto L_HasSkillThrowStone;
mes "You don't have Throw Stone";
close;
L_HasSkillThrowStone:
mes "You have got the skill Throw Stone";
close;

Example 2:

if (getskilllv(28) >= 5) goto L_HasSkillHeallvl5orMore;
if (getskilllv(28) == 10) goto L_HasSkillHealMaxed;
mes "You heal skill is below lvl 5";
close;
L_HasSkillHeallvl6orMore:
mes "Your heal lvl is 5 or more";
close;
L_HasSkillHealMaxed:
mes "Your heal lvl has been maxed";
close;