Simplify string mangling in ifmaybeload().

• Use strlcpy() instead of strcpy().
• Use strlcat() instead of a strlcpy() with a magic number subtracted from the length.
• Replace strncmp(..., strlen(foo) + 1) with strcmp(...).

Differential Revision: https://reviews.freebsd.org/D1814
Reviewed by: rpaulo
MFC after: 2 weeks