lib: Substitute some STREQLEN with STRPREFIX

lib: Substitute some STREQLEN with STRPREFIX

There are few cases where STREQLEN() is called like this:

  STREQLEN(var, string, strlen(string))

which is the same as STRPREFIX(var, string). Use STRPREFIX()
because it is more obvious what the check is doing.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>