change non-alphanumeric characters into '_' uprobe symbols

Just get rid of first non-alphanumeric character in uprobe
symbol to '\0' is not enough, Go program symbol name maybe like
'github.com/foo/bar.foobar', it will corrupt to 'github', it's
impossible to intepret.

So now the rule is simple: all non-alphanumeric characters
change to '_'.

A another method to fix this problem is change kernel event
name convention, let kernel tracing infrastructure accept
these event names.

Reported-by: Daniel Morsing <daniel.morsing@gmail.com>
Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>